Paperback for novels, hardback for nonfiction. Then again, most of the novels I read are relatively short. I don't think I've read any novel longer than 500 pages in years.
That's a shame. A good epic novel is a precious gem . The best ones always kill off the main characters slowly after you've had plenty of time to identify with them. A few of my favorites: An American Tragedy, Lonesome Dove, War and Peace, Pillars of the Earth
You are right. I hate my (relatively) short, modern attention span. When I was a kid I could just get lost in massive books, but now it seems there's always too many distractions.