Campaign mode's, I didn't actually finish WaW, don't like the WW2 guns especially seeing as they made the only WW2 gun I did actually like, rubbish (Kar98K) Add that to the grenade spam of Veteran I didn't play it much. MW2's story, while filled with plot holes and not making any sense, was just like a high budget Hollywood action film, so kept me entertained.
Competitive Multiplayer is won by MW2 aswell. Better guns, more kill streaks, and a load more things to level up. Although the kill streaks do unbalance it a lot, my friend got 81 kills to 19 deaths the other day.
WaW destroys MW2 in terms of Co-operative multiplayer. I haven't unlocked all the Spec-OPs yet, but I don't think there's a 4 player survival one with infinite rounds is there?