Paperback. While I'm a sucker for a good book I'm not sucker enough to spend $30 on the hardcover version. While the trade paperback while no doubt be rough around the edges in a couple years, it won't murder my wallet.
Hardcover is useless unless you carry that book around a lot or if it's a large book. I bought two Junji Ito hardcover manga last year. Everything else I bought was paperback including textbooks
Paperback is fine for smaller books, but I have a paperback Sherlock Holmes compilation that I got years ago and I opened it too much once and cracked the spine, so overall I'll have to say hardcovers are better.