Comparing both games in their prime;
Guns; mw2 (better balanced, though smgs should get a range P and the Ump had way too low damage drop off in mw2, being a 3 hit kill across the map)
Perks; mw3 (this killed the game in mw2 and you can't even call it a shooter do to DC Pro tubes and MLC knifing which is both game ruining / uber annoying, this said I had fun with explosives in mw2).
Killstreaks:mw3 (specialist is awesome, support mweh, but then again if you get a high score you can spam it more often than shi-t players)
Maps; mw3 (I love small-mid big and hectic, plus a lot of mw2 maps have a strange sand storm or whatever it's, it's annoying).
Hit detection & matchmaking / game performance online; mw2
Player movement; mw2 (getting stuck and not being able to go over anything basically is pretty fckin annoying mw3).
