Oh missed this!
Personally I prefer L4D1, much darker and sinister. Some of the named infected Steam introduced in L4D2 are a bit silly (The Jockey springs immediately to mind). Also the characters in L4D2 - Ellis and Coach are fun to listen to - but Nick? He's boring as hell. At least in L4D1 all the toons had personality (I know they're all the same, but when you have to listen to them it is better if they are actually interesting/funny).
Anyway, if you're planning another evening of zombie killing, please post!
I, obviously, have both games.