I'm trying to decide between: Magi of Hermes, Lords of Men and The Church. Opinions on which would be best?
I vote for Magi of Hermes.
- I love MoH: Plenty of spells and item ideas. Gives you a real feeling for the game. Oh, and older characters take so long to build. It's like short stories: Lots of little jewels.
- I think that Lords of Men is mostly a book about the medieval upper classes, and there are plenty of books about that.
- I haven't read the Church, but from its table of contents it seems to be an attempt at mechaniting faith. Strange.
Are you primarily a player or a storyguide?
What's the purpose of the PDF, for you?
Vrylakos
So I guess to be more explicit: story ideas? rules mechanical stuff? inspiration for characters? stuff to fit into your existent saga? etc. etc. etc. Fun Read?
For mechanics, Lords of Men is incredible. Mass combat, sieges, rules for horses, falcons, calling in favors, and more. I love it. I have seen MoH and it looks awesome but don't own. Similarly, do not own the Church.
The above question is the most important... looking for good reading or mechanics?
Really, it's a testament to the Ars Magica 5th Edition line that every time a book comes out, I am tempted to nudge my own saga to accommodate it or otherwise change the saga's focus to say, spotlight the Nobility, or add a monastery nearby, etc.
Vrylakos
There are reviews for all in the forum. Search and you shall find 
+1. Just added a priory to my saga, after reading The Church.
But yeah, the three books are so different it's hard to tell without additional information...
It really depends on what kind of saga you want. What things you feel you need help ti implementing, if you want.
While not inititally impressed by The Church - don't get me wring, it is good writing - it just wan't very relevant. But in time it represents one of the things I think we miss and need in out sagas. They tend to be very isolated and more Mythic than Europe. I have yet to implement anything, but I fully intend to. Likewise I wan't impressed by City & Guild but suddenly saw a great use for it while building a Redcap who had Touched by Realm and made magic clothing/travel equipment. I did end up re-building her without this, but only because she took as much time to keep track of as a magus. And I already play a magus and have almost half the Sg load...
Lords of Men gave me a lot of good stuff for whisking up plausible nobles and relations. Something - as above - we lacked before but intend to implement in an upcoming saga.
Magi of Hermes seems fun, but sadly I've never delved deeply into it. I know the magi in there have soe really cool spells and device effects, good for inspiration and so on. If you want to up the fantasy element and use old magi and powerful effects then buy this one.
But why not get them all? 
Your continued support keeps the line going. And who knows what gems will be published next year?
basically depends on ther approach: church, nobility or Order of Hermes. I have used Magi of hermes to run tribunals and to flesh out covenants fairly successfully, as well as for introducing potential enemies, rivals and allies. And parens (and aquaintances) to the characters.
The church deals with the institution. For the Divine there is the ROP book. Rules for peregrinations and to bukild institution "covenant-style". For me the best part was the politics part.
Lords of men is about the nobility, but includes some stuff on "mundane life for the peasants that serve the nobility" as well. To me it did not bring me much info that I didn't know already, and found it a 5th edition version of the mundane parts (read: most of it) of Heirs to Merlin. However, if you are into massed combat in your saga, and you like detailed combat in general, LoM is quite nice to have. even a "must have". We are the opposite (we have simplified combat since we find Ars too complicated in that field) so I do not use this book much. However, another player in my group find this to be THE book for Ars. he plays a minor noble, as you could guess
All books contain story seeds.
Both of these 2 last books might feature more prominently in our next saga, since we are going to play hedgies exclusively. TOur just finished saga has been high magic and hermetic politics, and we are moving away from that. being on negotiations with dragons and Might 70 faeries ends up being a little bit too much for our suspension of disbelief, so we are removing the order of hermes as we know it from the game.
Cheers,
Xavi