All things being equal (money, space, etc), would you rather own copies of the books you read? Or borrow them?
It depends. If I like it, then I want to own it. If I don't like it, then I never want to see it again.
I'm not very much of a re-reading person. I do re-read stuff every once in a while, but not often enough that I need to own anything.
I don't mind borrowing books. I obsess about keeping the books I own like new. If it's from a friend, then I take really good care of it. If it's a library book, then I still take care of it, but I don't mind placing it on a not completely flat surface or dog-earing it.
I can usually tell if I'll like a book before I ever open it, so that helps me with picking what to buy and what to borrow.
Overall, I prefer borrowing books, unless I can tell that I'll really love it.