Both games look extremly great to play and took a great effort to make, But I have to Say left 4 dead because I truly love bashing zombies and maybe amounts of Gore!
I played L4D demo its pretty cool. Fallout 3 i reserved which is awesome. Matters what ur into more shoot em' up l4d rpg with shooting fallout, fallout has much more depth and will keep u busy for many hours, L4D probably will not hav much depth but fun game, if I had to choose F3. Plus i hav about 23-24 hours in the game and i have just scractched the surface, and one more thing, in the future they will have DLC for F3