Big fan of both MW1 + MW2
MW2 is slicker, and the games are very short, everything feels like a sprint, I am used to 20min a round on MW1, but MW2 it is always very quick.
What MW2 does do is bring the collector out in me, I am constantly trying to do silly manoeuvres (such as sticking a semtex to someone so as to blow it up next to some one else) so that I get an xp reward, which is pretty pointless, but they reward me with a shiney badge next to my name. All MW1 gave me was a golden gun after hours and hours of constant headshots.
What ever you do don't buy WaW.
I did not get on with MW2 for the 1st week, but then I got to understand it a bit better. Fine you don't get dedicated servers, but you also don't end up playing against some lagger from New Zealand who thought this was a good server to try.