As a new Ars Magica player, looking at the new vista of books to buy but saddened by the insane price tags for used copies of OOP books on Amazon, allow me to vote as follows: softcover, hardcover, whatever, I just hope to see these books reprinted soon!
I guess one of the features some people are considering is how long these books are going to have to look.
Take my battered wizards grimoire... I have had that book for a long long time and it still gets glanced at now and then. Mainly for ideas.
Some of these books are likely to be in use for many mny years to come. 5th Ed. has been around for a few years now and much older material is not reprinted into it and quite possibly never will be. mainstream supplements about houses and characters we all use will for sure. I suspect 15 years from now or even 20 years from now arms magica may be on its 7th or 8th edition, but I doubt art & academie will have made a new edition in that time.
Hence, I would prefer as durable a version as possible.