Probably been asked a million times before but it's a simple question, so which do you prefer?
One is cheaper and easier to carry around, one is a bit more pricey but looks nice and lasts longer.
For me it depends, if I'm going to be toting it around everything then paperback of course, but for the books I like and have time to actually sit down and read I prefer hardback, which also last longer(I have one from 1911, so they certainly stand time well enough) and look better on the shelf.
I usually find that with a good book, it's worth the extra money for something you know will last you.