MW2's multiplayer is just sick. It's the most fun I've had playing a game in a long time. The single palyer is decent but I was hoping for a little bit more.
Uncharted has a great story mode but the mutliplayer is no where near is as addictive as MW2.
It's like comparing apples and oranges...it all depends on what i'm in the mood for.
But I see myself playing MW2 for months, I can't say the same for UC2. So for that reason I give my vote for MW2.