If you're into the more traditional online multi-player, Modern Warfare 2. L4D2 is quite amazing if you're into co-op and have actual friends to play with, but it's style is not for everyone. It's also kind of a drag if you don't know anyone who plays it. I'd also hate to play it if you're only using the 360, because Versus mode would be terrible w/o mouse control. (as infected, mainly)

I'm still playing L4D almost a year later, mostly Versus mode, with literally hundreds of hours logged. (and the Zombie Genocidest achivement unlocked...woo, 53,000+ killed. Modern Warfare does nothing more than take up hdd space at this point, despite being an excellent game. It just didn't hold my long-term interest.

Needless to say, MW2 will likely be the more fulfilling single-player experience. L4D's sp is mostly training for online play. Honestly though, who plays L4D by themselves? What a bore, and it's missing the entire point of the game.