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, I would probably fill my library with handsewn, first-edition hardcover books. Why? Because, as Frank Lloyd Wright said, "Give me the luxuries of life and I will willingly do without the necessities." I have a sensuous love of luxurious items such as beautiful books. This kind of books would add to my reading enjoyment.