--Hardcover? Or paperback?
--Illustrations? Or just text?
--First editions? Or you don’t care?
--Signed by the author? Or not?
For me, and in all of the above cases, it depends on the book and why I want it. For some books, I've gone out of my way to purchase a specific edition; I bought the illustrated hardcover copy of The Silmarillion several years ago, because I wanted it as a collectible. (Not that it's worth anything, but it's special to me.) For most books, though, my primary focus is cheap, so I don't really care which version I get as long as it doesn't cost too much. There's a part of me that likes the idea of a library full of beautiful hardcover books with decorated spines. But then there's another (and somewhat more practical) part of me that has grown to love my menagerie of disintegrating books with the variety of cheaply colored covers. It's kind of like my teacup collection: I don't think I want a full set (of teacups, that is...), because I think I like for them all to look different.
I also write in most of my books, because I like to underline things that jump out at me (I'm a kinetic reader!). So, I don't want to spend a fortune on an edition just to mark it up with a pen.