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?
Paperbacks. I have small hands and hardcover are harder to hold. And I am very nearsighted, so when I read in bed, without my glasses, I have to hold the book very close to my eyes, and paperbacks are much easier for this reason. Have you ever dropped a hardcover on your face? No fun.
The only advantage to hardcover is that they are released first. I concede they are prettier, but only the ones with the cloth actually covered. I don't find the dust cover helps much and I always remove it to read the book, and then my book looks all anonymous and like I am trying to hide what I am reading. And I then I have piles of dustcovers flopping around the floor of my bedroom. But hardcovers look nicer on a shelf.
So, it's softcover to read, and hardcover to look at on shelves.