They both have some really nice points, and some less-than-good points, but because they are essentially the same game, I find I play a lot more of one than the other...
I prefer the first game due to the darker and more urban campaigns, much more tense experience. I also prefer the characters from the first game.
However, I prefer the second one due to the extra items, melee weapons, ammo types, boomer bile, defib, adrenaline, and just plain out more guns. I also like the extra playable infected in versus, and would especially miss the charger and spitter. And recently, realism versus is an epic mode only in 2.
Simply, I play the second over the first for a) the fact it has a lot of additions, and going back to 1 (while improves a little) adds nothing specific, and takes away plenty, b) I usually play when invited, and it's always to 2.
I would absolutely love it if the original 4 1/2 campaigns could be transferred to number 2, incorporating the new items and infected (and making them ALL playable in versus) and keeping it balanced etc, rather than just a simple port.
To get back to the question at hand, which do you prefer, and why?