People are bandying around Potent Magic again, mainly as a way to get around House Tremere's restriction on magical focus. However, I have quite understood the appeal of Potent Magic. Perhaps it could be explained to me.
A sample Minor Magical Focus could be "Canines", as a tiny subset of Animal. A mage with this virtue can get their lowest Art score doubled when magically affecting dogs.
A Special Circumstances in "Canines" allows a mage with this Virtue to gain +3 to Casting totals when magically affecting dogs.
A minor Potent Magic in "Canines" allows a mage with this Virtue to gain +3 to Casting totals when magically affecting dogs, using only specially invented spells with the limitation certain ritual activities need to be performed.
The +3 from potent magic applies to all spells affecting canines. But, when you invent spells specifically for your potent magic, you can incorporate shape and material bonus, and get an even higher casting total.
Potent Magic grants a +3 bonus to Lab Totals and Casting Totals.
Special Circumstances grants a +3 bonus to cast and resist spells.
So their respective bonus apply to different things.
Where Potent Magic really shines is the possibility of inventing Potent spells, which grant an additional bonus when casting them (Potency score) but require the use of one or more Casting Items. Anyone can learn those spells, but only a magus with Potent Magic can invent them.
Edit: Note that the bonus from Potent Magic stacks with the Potency score of a Potent spell. So in your example of a magic with Minor Potent Magic with canines, who invented a Potent spell that controls canines with a collar as a Casting Item (Potency score of 6), will have both the +3 and the +6 bonuses added to his Casting Total for the spell. The negative side is that he cannot cast the spell if he doesn't have a collar.
I always like to bring up as well that potent spells would make good trade goods. If you want to purchase a spell to do a thing and one covenant is happy to sell you the basic spell and another covenant is charging a little extra, buttheir spell gives you a +6 bonus onto the casting total, wouldn;t you fork out an extra pawn of vis or two for the upgrade?
This is especially the case where the spell is a) a ritual - lots of time to prepare means you will almost certainly have the required item when casting, b) something for which you are almost certain to carry the item anyway - e.g. fire magic with a ruby as the required item. What fire mage doesn't have a ruby in their talisman already? How hard is it to get a ruby in a ring? or c) when you really need all the extra penetration total you can get and are willing to prepare. e.g. commission a spell especially from the potent magic: Dragons guy to defeat your arch-nemesis dragon.
