Well Uncharted 3 is much better, but here is the breakdown
Campaign/Story
Uncharted 3 was great, really really great... I really like all the characters, though the girls were under used, and the way it explored the relationship between Sully and Nate was great. The pacing was also good, though not quite as perfect as Uncharted 2 was.
Gears 3 was decent, but still not quite as good. The story itself was mostly good, but the dialogue was annoying, and often innapropriate given the sentiment that the cutscenes were trying to get across. I just feel that they did not properly combine the story telling in the cut scenes and gameplay, the seemed totally at odds for much of the game. And the bosses were quite annoying, as were some of the enemies, but that was really my fault for playing on insane...
Graphics
Well Uncharted 3 is just much better, by some distance, there isn't really any argument here. Gears looks really nice, but doesn't have the same number of effects with thinks shaking round and water and sound physics, which are just wonderful in Uncharted 3
Co-op/Multiplayer
Well, I think Gears will win this, but I can't really say as I haven't played either multiplayer, though I have played campaign co-op in Gears (twice), and I do love Horde, so I feel that will win
Gameplay
Naughty Dog have really upped their gunplay. I didn't have any problems with the shooting mechanics, and the melee combat has been improved too. I think it feels great, the way it is a bit button mashy is great, it feels really appropriate for the situation. I also loved the gameplay on sliding things that were falling apart. And the near last levels were great, though I don't want to spoil what they were about
The Gears shooting is very nice, but I didn't like the annoying enemies of this game, again I don't actually want to spoil them. There was some nice variety to the environments, and there were different types of enemies to shoot, but it was a little repetitive, but still all fun, so this category is probably a draw
