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?
Strangely, this is a hard question to answer! Hardbacks seem more durable, but I find trade paperbacks (not mass market paperbacks) physically easier to read. On balance, I think I’ll say trade paperbacks, because they take up less space and I therefore have more room for books!