I prefer L4D2, because it, as previous posters have stated, improves on just about everything.
The levels are greatly varied, with both daytime and nighttime levels. It has more weapons, not to mention the different kinds of ammo (which has a tendency to mess at least MY party up.).
The characters are still pretty interesting, at least Coach and Ellis, and the three new special infected are all great additions.
Not to mention that it gets HARDER, which was needed. Although it's even TOO hard for myself and the people I play with, but that's a different story.
I would have preferred if this wasn't a sequel, but rather an add-on or DLC to the first game, since it's basically the same thing, albeit improved. That said, I still love it.
Anyway, L4D2 hands down.