Most multi games look better on xbox 360 because it is easier to make games on it. Even 5 years into the PS3's life most devs still cannot get it right. The reason why PS3 exclusives look way better than practically every game that has ever existed is because those devs KNOW how to make a game for the PS3 and they are also not limited by the xbox 360.
COD MW2 is the same on the 360 and PS3. The only differences is the uncompressed audio so you get better sound but the 360 has better FPS on average. Call of Duty black ops runs, looks, and sounds worse on the PS3 since treyarch is a crap dev.
Basically the games all come down to the developers. If the dev is great you get great graphics and gameplay.
Epic games knows the 360 very well which is why gears 3 looks almost as good as uncharted 2 and it is on the 360 but then again uncharted 2 will probably have way more things going on. If you have played the train level, building collapsing level, etc... then you know what I am talking about.