I tend to buy more, just because I re-read books over and over and over again. As stupid as it sounds, I get very attached to them. The creases on the spine, where I've turned a page over - all hold memories for me. Its a personal thing. I love the smell of a fresh new book. I'm also building up my own library. So far I own around 500 or so. I've given away 12 in my lifetime that I didn't like.
I do also borrow books from the library (and if I like them I buy them) - but not so much recently. Ours isn't that well stocked and I really hate books with god knows what between the pages.
However I am a big supporter of libraries in general - I think they are wonderful and I wish the books there were better taken care of and had a bigger range.