I like MW much better, for a variety of reasons. First off, as a history major, I'm almost obsessive compulsive about my shooters being at least moderately realistic. The single player in WaW was very good for this, main character's mutant healing factor nonwithstanding. However, in multiplayer, its a huge curse. In a game which HEAVILY favors the sub-machine gun classes, using a bolt action rifle like 90% of the soldiers in WW2 would have been doing gets me killed alot. But, thats just a minor historical hiccup that cannot be rectified in multiplayer, so i'll ignore it. Now comes the real problem. I've already mentioned it, but the sub-machine guns are incredibly overpowered. The situation with the MP5 was bad enough in MW, and instead of learning from the experience, Treyarch went entirely the wrong way and made the MP40 more of a game winner than the MP5 or the M16 are in MW. The SMGs are both too accurate and far overpowered when compared to their real-life counterparts, and thus you always end up in games where you might be the only player not using an SMG...which gets annoying extremely quickly. There are a few good things I'm willing to say about WaW, though. The mechanic in which you hafta EARN your sniper scopes is a good idea, and treyarch heard at least some of the complaints, and made grenades, and also martyrdom, a lot less effective. The assist point system is also a plus, but it occasionally makes me angry when i get +8 cuz some dumbass nabbed my last stand kill.
Because of these reasons, I enjoy 4 faaaar more.