All other things (like price and storage space) being equal, given a choice in a perfect world, would you rather have paperbacks in your library? Or hardcovers? And why?
I rarely purchase fiction books anymore. I use the library lots. So for a library book I prefer hard back as those tend to be in better shape than the paper backs. Although I love a good book, I enjoy audio books too.
I do purchase reference books especially if it has to do with my fiber addiction: crocheting, knitting, spinning etc. Those I like something that stays open more easily but can take some wear an tear too. So usually a heavy duty paper back. Hardback pattern books get heavy in my project bag.