I don't think you can really compare gears 2, and Uncharted2 in that way.

Gears uses unreal Engine 3.

That engine performs better on 360, almost every multiplatform game using it either runs faster, higher res, or just plain nicer on the 360.

Uncharted2 is a custom PS3 engine.

I don't think they can ever claim UE3 pushes the 360 the the limit as a non-generic, custom engine could.

But again, it does come down to design efficiency.

Uncharted 2, I would agree looks

"better" than Gears 2.

But it's hard to know if that because of clever design, or a technological advantage.

Is uncharted 2 pushing more triangles, pixels, doing more animation? Or is it just nicer art?

Do their artists have a better grasp of making the most out of each polygon,

(regular clothed humans need a lot less polys than a body armoured COG, especially when not right up close) and maintaining consistent polygon density and scene complexity, so the engine actually doesn't need to push as many polys and pixels?

It doesn't matter to the end user.

They just see that Uncharted 2 looks better than older stuff, but to claim it shows a significant technical edge of PS3 over 360 isn't a provable conclusion.

EDIT:

Re COD4.

It's also worth noting

than leading development on PS3 is not far of saying using PS3 as the lowest common denominator.

Designing around what they could get out of PS3 might mean the 2 versions end up mostly the same, but it can also mean the 360 version has been held back to make that so.
