The art, type of cover, cost, and book all come into play when I'm buying a book. I don't really have a preference, since I'm all about visuals when it comes to books. While paperbacks are cheaper and easier to carry around, I'd probably go for a hardcover if the cover looks nice and I really want the book. I generally stay away from oversized hardcovers, as they're way too bulky and I can't really throw them into my bag for reading outside of the house. Book jackets also tend to bother me, especially since I can't read with those on the book. I hate putting my jacket somewhere and having someone smash it.
There's this one type of hardcover that I'm crazy about. When you tap your nails on the cover, it makes a nice hollow sound. It also has a nice matte finish to it. The only problem is that scratches show up on those.
I was recently obsessing over this wonderful Macbeth paperback, which features a gorgeous cover. It looks much better in real life and features a very nice finish.
Jam it back in, in the dark.