I too have a strange affinity for used books (and not just paperbacks); they seem to hold a little bit of extra story somehow. It holds one, and it has one. The book itself has a history. Right now I'm reading a copy of From The Land Of Fear by Harlan Ellison published in 1973. I will almost always buy an old used copy over a new one, unless it's not an option for whatever reason, or I'm being impulsive.
I can't stand to read anything of length on a computer screen. If it's more than three or four pages, I want a hard copy.
Other than that, for me it depends on the fiction. My old standby is always a trade paperback(It's the perfect size, and hardbacks are too heavy to lug around all day.), but there are some things I'd much rather hear in audio. I Killed Awesome Man is a great example. I'm not big on superheros, and if I saw a copy of the exact same book in print I probably wouldn't have read more than a few pages. It's a little too comic-book-action-y for me. But since many (okay, most) of my favorite voices collaborated on it, I seem to enjoy it more.
I also usually prefer any kind of comedy in audio, because delivery is so important. Of course, there's much more that can go wrong with audio fiction. Bad sound quality or a bad reader can absolutely ruin an otherwise wonderful story. On the other hand, a talented reader (we're lucky to get lots of those here) can breathe life into a story I would have otherwise found kinda mediocre. At that point it becomes more of a performance than a story though.
