I definitely won't be picking either option. Fallout 3 is leaps and bounds beyond L4D. Don't get me wrong, L4D is a decent game, but not a GOTY. It's just too one sided, IMO - get from point A to point B, and don't die.
This poll is just a tad biased but I would say it's one of the best games I've ever played. As a die hard Fallout 1&2 fan I would agree that Fallout 3 is a good game, but I thought it was a bit too much "Oblivion" and not enough "Fallout". Also, once I decided to go completely evil and kill everyone I came across, the dull landscape became that much more cold and lonely. I also thought it was a bit rough around the edges.
I'm getting off topic. Anyway, for anyone who doesn't get why Left 4 Dead is so awesome, you haven't played with the right people yet. (Go join the Steam group!)
Oh, and Le Sac, your avatar makes me want to go watch IT Crowd.