Weapons- While I'm sure Resistance will have the cooler weapons, nothing beats a shotgun when blowing a zombies head off. Left4Dead
Enemies- Endless waves of zombies always win. Left4Dead
AI- Well since Resistance is primarily a single player game and Left4Dead is purely multiplayer with only mindless zombies I guess Resistance 2 gets this one. I know a lot of people are talking about the director, but I don't lump that in with the AI (even though I should), I put it in its own awesome catagory.
Art Direction- Resistance 2, but only because it's hard to make a new interesting zombie art direction. In my opinion they're both pretty boring.
Replay value- Left4Dead
Graphics- Resistance 2
Sales- Left4Dead -- go go Steam
Overall- Left4Dead easily
