Oh thank God, I thought this was going to be a versus thread at first.
Let's see...I happen to have both and let me just say that I think that COD4 is better overall, but that doesn't mean that COD: WAW should be overlooked so easily. In my opinion COD: WAW basically took everything that was great about COD4 but just set it during WWII.
If you're going to be playing mostly multiplayer, you can't really go wrong with either one, seeing as how they are almost completely the same in terms of the ranking system and challenges. But there are a few changes to the weapons system obviously, like the sniper rifles. Instead of just earning a rifle with a scope, you have to complete the marksman challenges for bolt-action rifles to recieve the scope attachment.
Also, COD: WAW has tanks.