I try to support my favorite authors as much as I can, so I will usually buy a copy either at release or when it hits paperback.

I tend to use the library for books or authors I am unsure of. If I like it and think I will reread it I will buy a copy. I've recently downsized my personal library cause I just, sadly do not read as much as I did about 10 years ago. Books were just collecting dust.