The issue of what happens to an object changed by Muto when the spell ends has been discussed before, e.g., When a Muto effect ends...
In general, people seem to agree that if you turn a stone to water and then carry that water away in four pots, when the spell ends you have 4 pots of stone. If you left the water on the ground, you'd have a puddle-shaped piece of rock.
Given this, why would a Terram magus ever learn Perdo? To destroy a rock is base 4. To turn that rock into water is also base 4 - yes it will instantly change back to rock, but in the moment it was water it will have collapsed and spread onto the ground, essentially destroying it. You achieve the same effect at same level. Even if it was one magnitude higher to destroy the rock with Muto, it is still best to develop Muto and entirely ignore Perdo, given all the other things you can do with Muto while also being able to destroy stuff.
Conclusion: Perdo is worse than Muto with Terram (Potentially applies to other forms too).
Thoughts on this? My main thought so far has been to house rule that any Muto effect that compromises the structural integrity of the object has a Perdo requisite, but I'd be interested to hear any resolutions that don't involve house rules.
EDIT: @ErikT pointed out this needs an Aquam requisite, which is a good point. I've reformulated it as turning stone or metal into sand, which can destroy armour, weapons, buildings - most of the things a Perdo Terram magus would want to destroy. Muto Terram can actually destroy these things at lower mag, if you think a Mom. duration into sand would mean the thing falls apart - or at worst, at the same mag, if you think it would take Diam. for them to fall apart.
Sure, you could turn them into water, but if you want to dig a tunnel, what you'd end up having is an increasingly shorter tunnel, as the rock you Muto turns to water, spreads out, and starts accumulating layers, whereas with Perdo, it's just gone. This is especially true if you tunnel downwards. Muto would be defeating the purpose.
Yes, tunnelling downwards would not work. Tunnelling into a hillside could arguably work, as the water will spread out and leave the tunnel and then turn into a layer of rock floor for the tunnel - it would still be a tunnel.
And this is a very specific case. If my answer to a player's question, "Why don't I just learn Muto?" is "Because then you can't create tunnels going downwards," well... I'm not sure that's gonna convince them they need Perdo
Also, if you use a Muto spell with Momentary duration, then if you turn rock into water, the water won't have time to collapse or spread out much until the duration ends and the water turns back into rock.
Even with a lenient interpretation the water won't have time to flow out of a tunnel one is digging, but will just stay on the floor making for a quite uneven floor which grows ever closer to the ceiling as that water turns back into rock.
Increase the duration and you need a higher level spell.
Using Muto to turn rock into water would also have an Aquam requisite, while just destroying the rock with Perdo won't have a requisite.
I'm not sure why the water would turn into rock that grows back to fill the tunnel? I suppose it would if you use the interpretation that when a Muto spell ends the object returns to the original shape, but that creates a lot of problems such as water teleporting hundreds of miles to become part of the rock it used to be, etc. I would see the water turning to rock wherever it is when the spell ends, and staying there, spread across the ground - not growing in size.
That aside, the requisite is indeed a good point that makes this harder. But I guess I could turn stuff into sand, which would not need a requisite and could still destroy many things, e.g., towers, statues, armour, weapons. When I turn the knight's armour into sand, it will fall off him and will not return onto him when the spell ends (in the way I see Muto commonly interpreted).
Even if you rule that it takes concentration duration to allow time for the sand to fall, that is still only 1 mag higher than the Perdo guideline, which still seems to make ignoring Perdo and focusing on Muto the best option by far.
EDIT: I've just realised that with conc. duration changing metal or stone to sand is actually the same magnitude as destroying it with Perdo! Therefore, if the sand is deemed to fall away immediately with a momentary duration, it's actually easier to destroy armour, weapons, buildings etc. with Muto than it is with Perdo. If conc. duration is required, it's the same.
[quote="Whitescar, post:5, topic:169194, full:true"] I would see the water turning to rock wherever it is when the spell ends, and staying there, spread across the ground - not growing in size.
[/quote]
Sure, but it would still take up the same volume of space as the original rock - just in a somewhat different place.
If you are digging a long tunnel, then a Momentary duration spell would certainly not allow the water to flow out of the tunnel, so you'd still have the same amount of rock in there.
Use Muto spells with a longer duration and it could work - if the tunnel slants upwards, but you'd end up with an awful lot of stone right outside the tunnel.
If you want to destroy a tower or similar, then using MuTe to turn stone into sand should work just fine - unless one rules that Momentary duration is just Momentary and only lasts a moment, in which case the sand won't have time to go anywhere before it turns back into stone.
With a longer duration it would work great - but a Perdo spell would work just as great without needing that longer duration.
So to answer your original question - no, Muto Terram is not strictly better than Perdo Terram, but in many cases it can be used as an almost-as-good substitute.
Except see my edit above - I made a mistake, in the current guidelines, destroying armour, weapons, towers, buildings is actually the same difficulty or easier with Muto than it is with Perdo (Destroy stone is level 4; stone to sand momentary is level 3, and level 4 with concentration). Given everything else you can do with Muto, I would say it is indeed strictly better.
The problem with using MuTe as a substitute for PeTe is that when the duration ends you still have the same amount of rock as before, just in a slightly different place.
For some applications (destroying a knight's armour) this is not much of a problem, but for other uses (digging a tunnel) it can be a quite signficant problem.
There are also some fun uses of PeTe that just won't work easily with MuTe.
Ex. Using PeTe to dig a large hole just under an enemy magus should work just fine for getting him to fall down in it. Using MuTe to turn the ground under him into sand won't have the same effect - his Parma Magica should prevent him from sinking down into the (magical transformed) sand.
So while you do have a good point that MuTe can often be used as a substitute for PeTe, while having many other uses as well, there are a bunch of cases where PeTe is the better choice.
But I agree that given the choice it is far better to specialize in MuTe than in PeTe.
Two points to consider: Speed, and magi don't just use their preferred TeFo combination all the time.
Speed - If you want to create a pit under someone and drop them in it (let's face it, Pit of the Gaping Earth is a beloved combat spell as it allows you to deal with enemies no matter their magic resistance by targetting the earth beneath them) you don't want to be waiting around for it to develop. If you're attacking a rival's tower, you want to make a breach in the wall right now as each round it takes for the hole to develop is another round anyone trying to get in the breach gets attacked.
Magi don't just use their main TeFo combination - yes, MuTe is probably more versatile than PeTe. However, if you magus has maxed out on Pe as a starting mage, then they can quite easily have a range of other effects such as Veil of Invisibility (PeIm) or removing people's memories. A MuTe specialist would instead be using disguising MuIm and altering memories with MuIm. Magi are likely to choose spells around what suits the style of their character, so plenty will prefer Pe to Te.
Better may be, but strictly better would imply, to me, that there exists no situation where Perdo can produce a more desireable result, and we have already proved that false. When you dig a tunnel, and especially if you dig a shaft, using MuTe, you need another means to dispose of the material. Even if you are digging uphill, so that sand and water can flow downwards, you might find yourself standing in it when it turns solid.
I agree with you that Perdo is not the most versatile of techniques, but it has a couple of uses which are hard to replace. Veil of Invisibility and Demon's Eternal Oblivion spring to mind, in addition to the straight forward and effortless tunneller application.
I would also argue that the PeTe attack spells (Pit of the Gaping Earth IIRC) are more effective than MuTe equivalents, because the target would fall immediately with a momentary spell, with no risk of floating until the spell ends. As @darkwing points out, MuTe may be too slow in a critical situation.
This is exactly the problem that my party has encountered. The Party's Te mage took the faerie based flaw that his Pe is temporary, so instead of just clearing the underground rooms 10 paces cubes at a time, he made a ring of MuTe rock to gems (so that it shrinks) with Sun duration and then the grogs have to rush back to the surface before sun down with bags of temporary gems.
Perdo, the armour is gone. Only the most generous SG will allow turning the armour momentarily to sand do anything but a tiny degree of cosmetic damage as the sand starts to fall then turns back to metal.
The moment you need concentration/ring duration, you are spending valuable time or needing higher magnitude.
Speed doesn't actually seem like a problem at all. As I've pointed out, turning the armour to sand with diameter duration is the same magnitude at destroying it. And surely the time it takes for sand to fall from a person is a few seconds at most. Besides which, once you cast a diameter spell and turn the armour to sand, it's already sand so it's not stopping any attacks. Muto and Perdo are therefore no different in respect to the speed of destruction.
For getting rid of a knight's armour you are correct that there is no real disadvantage in using MuTe to turn the armour into sand compared to just destroying it with PeTe.
For doing things like making a hole in a castle wall, going the MuTe way will turn a section of the wall into a big pile of sand that you won't have time to clear away before the duration ends - unless you use a long duration that will increase the magnitude of the spell significantly - and no matter how you do it, it will take time to get past the sand.
With PeTe you end up with a hole in the wall that you can just walk in through, with no delay at all.
PeTe to destroy opacity and create glassteel- if done with Muto the effect is temporary. Your analysis is based on the assumption they will be clever about using Muto and straightforward about using perdo instead of the other way around.
Yes, indeed, there are creative things that can be done with Perdo. And as @ErikT and others have pointed out, it's a better way to make tunnels or a hole in a wall. Still, these all seem like rather situational benefits, whereas the general destruction of metal and stone things like weapons and buildings seems to be an obvious and primary use of Perdo Terram, and yet it's no better at it than Muto. I still think a Perdo Terram focused player would be pretty annoyed watching a Muto Terram magus destroy weapons, armour, rocks, and buildings just as easily as he does, and then make statues come alive and attack people as well.
At the same time a MuTe specialist would be depressed seeing a PeTe specialist create permenant stone windows or floating castles (destroy weight) and so forth. Maybe the best answer here is to be a terram specialist rather than a PeTe spcialist or MyTe specialist. But is MuTe strictly better? No.
I'm not sure why you keep repeating that destroying is as easy with Muto Terram than it is with Perdo Terram. It isn't. A Muto Terram spell will always be at least one magnitude higher than the Perdo Terram spell to include a non-momentary duration, and have undesirable side effects such as the described puddle of stone. It will often come with another art as a requisite, unless you're transforming say, Stone into sand, which if you were, you would be able to achieve an effect at another magnitude under with Perdo. E.g. weaken dirt, you could turn a stone wall into loose rocks the size of sand, and it would be permanent whereas the muto terram spell wouldn't be. Muto Terram is objectively worst than Perdo Terram at achieving the effects of Perdo Terram, unless the goal is a temporary destruction (e.g. transforming a wall into air for 2m), but even with such a temporary destruction, you would still have a MuTe (Au) of the same magnitude than the PeTe with a temporary duration, and thus rely on 3 arts rather than two for your effect.
Did you read the previous posts? I've outlined the reasoning very clearly. Muto terram base 4 turns stone to sand with diameter duration, effectively destroying any weapon, armour, building, etc. you care to target. Perdo destroys those things at base 4, so that is the same.
Yes, this is the subject of this thread based on my edit to the OP.
[quote="temprobe, post:18, topic:169194"]
which if you were, you would be able to achieve an effect at another magnitude under with Perdo. E.g. weaken dirt, you could turn a stone wall into loose rocks the size of sand [/quote]
Interesting - so you think that the "weaken" guideline is essentially the same as turning stone into sand? That would make it easier to destroy things with Perdo, by one magnitude, but given that the spell "Fist of Shattering" (p.155) breaks apart something and uses the level 4 base for "destroy," I am inclined to think that weaken only means to crack or split, not break apart or make into sand-sized chunks.
