It's hard to decide. MW3 is much more advanced in a way with Specialist and balancing. I love how most of the guns are good and hard to tell if it's OP or not when comparing. It's too noob friendly, however. Support and DMH (are they even going to nerf that shit?) really hinders the veterans who are just trying to enjoy the game. MW2 had much better maps. The guns were OP, and don't take that in a wrong way. Everyone uses the UMP, SCAR and ACR... which sorta makes it fair, I guess. Then you have Last Stand and of course, the dreaded OMA Noobtube combo.
So in the end, I'd rather MW2 because rushing is way too fun on that game with almost any gun given that you use it properly. But I'm forced to play MW3 because MW2 is hacked as hell! Plus, some people call MW3 "MW2.5". I guess I could agree to some levels. I love the Infinity Ward feel and I just can't help myself.