Its a toughy
Both games are superb but as two sequels with little in the way of new genre defining gameplay a think its a bit of a moot point.
MW2 is undoubtedly going to be the greater commercial success and I opt for this title in the end through no other reason than its incredible online gameplay.
Uncharted 2 is a fantastic game and it would win a graphics showdown with MW2.
Also being a multiplat MW2 garners a lot of cross party support.