Dead Space is a better game in every way (production values, graphics, story, etc.). Though, it doesn't mean that Left 4 Dead is a bad game (probably overpraised because it's a Valve game though). Left 4 Dead is a good "arcade style" game that has more replay value.
Both games have that scary atmosphere, but I think EA did a better job with Dead Space (don't worry, both games are not that scary! ;) ).
With all that said, it depends if you prefer a good single player or a good multiplayer game.
Random game thought :
Why is Bionic Commando Rearmed 2 getting so much hate? We finally get a real game and they're not even satisfied... I'm starting to hate the gaming community so f****** much...







