Both are great…Left 4 Dead I feel is a more engaging co-op experience because it’s two (or more!) against the end of the world. I love it! Anything by Valve is pretty much a guaranteed win, and I don’t think you’ll be disappointed. The online experience with pick-ups is frustrating because people seem to want to lallygag instead of bust through as intended.
CoD:WaW or even CoD:4 (I’ve heard better things about 4, but I bought both at the same time, so I get the two confused constantly) if you’re a wargame-fan. The weapons selection and specialization are typical and satisfying if you’re familiar with the CoD empire. I tend to fall back on it regularly, it’s my FPS home-base, but it doesn’t have the ohshitohshitohshit YES WE MADE IT YESSSS seat of your pants kind of feel that Left 4 Dead has. It’s way better for online multiplayer, but I found the split screen experience to be a bit disappointing.
You’re an FPS’er? awesome! We should join forces sometime and destroy things.