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?
All things being equal? Trade-size paperback. They still have the heft of a good hardback, without the weight of the hard covers. They seem to have better quality paper than the mass-market paperbacks, and don't have the jagged edges of the hardcovers. I hate those uneven edges - they look unfinished to me.