i would have to agree with what most people are saying here: mushrooms are far stronger than any acid that i've taken, and far more spiritual. the analogy of matrix (acid) vs. lord of the rings (mushrooms) is pretty good, although i might even take it a little bit further and say that acid is like watching a movie, and mushrooms is like reading a deeply spiritual book.
i find that acid is a great party drug. you can take it and have a lot of fun; dance, laugh, smoke, enjoy the music and the sights, etc. mushrooms can provide most of these experiences too, but they aren't as recreational. i've taken mushrooms on a whim for a friday night and paid a high price. caveat emptor.
also, a note about the 'mindfuck' qualities of the two drugs. i find that with acid, i get a definite sense of "here i am on acid, everything's fucked up just like it should be." you get weird thoughts, but there's a definite sense of continuity throughout the trip -- it's linear. whereas with mushrooms, there's a sense of "wow, this is pretty damned weird", but i find that there's no continuity. i find that mushrooms make me 'loop' more. i see circular patterns in my behaviour on mushrooms. i'm constantly thinking "well, a lead me to b, b lead me to c, and c has lead me back here to a. *aaarrgghhh*!" and of course, that can be equal parts funny, deep, and frightening.
as for preference, it's completely situational. if i was going out dancing (to anything other than psytrance/goa) i'd prefer acid. but if i was planning a day out tripping with some friends, i'd probably prefer mushrooms.
peace and be safe.