I'm gonna dissent and say Left 4 Dead 2. Then I am going to recommend that you discard all opinions on both. I believe it is totally unfair to judge either game before you have tried it. Borderlands looks awesome, but could be a complete and utter failure for what we know. And while everybody says that L4D2 should just be an expansion, I don't know of anybody who has played the entire game, so to judge its worth now is totally unfair.
Basically, what I mean is that you should only listen to people who have played both games. Right now, as far as I know, that does not include anyone who has replied.