I think Uncharted 3 to be a massively better game. My reasons:
1) Uncharted has characters I can care about far more, they show more personality and trigger emotion. Gears of War centers around some empthy headed testosteroned Space Marines. Awesome acting performances in the Uncharted series, IMO quite horrible in Gears of War.
2) Story telling. IMO Uncharted is worthy of hollywood in this regard, Gears of War's story is forgetable to say the least. Actually after completing Gears of War 1, I forgot the plot, actually even before that... Something about the fate of human kind on some foreign planet being in the hands in some peanut brained bodybuilders during some strange alien invasion. From what I have seen from Gears of War 2 it's more of the same and I think Gears of War 3 will be more of the same too.
3) Graphically Uncharted 2 looks way beyond anything achieved in Gears of War for example with regard to fluid animations/progress, details, lighting, special effects and physics. Even Uncharted 1 performed anti-aliasing combined with great HDR in high definition which no 360 game has been able to match.
4) Audio wise, the Uncharted series comes in multiple languages. So when I play through the campaign a few more times (on a higher difficulty level), I switch to another language like my native language Dutch and maybe German after completing the campaign in English. This actually keeps the story feel fresher to me.
5) Online mulitplayer. Uncharted 2's online mulit-player is a lot more varied and it's also available for free to anyone who buys the game! On the XBox 360 you need to pay extra each month/year to be able to play the game online and what's even worse you will have to bug your gaming buddies to do the same...
6) Gameplay mechanics. Drake is a lot more flexible with regard to movement than Marcus. Drake can roll dive, hang from the edge of a building while aiming and shooting. You can perform various pushes and kicks. Marcus can unrealistically crouch run (I'm a physiotherapist) but is unable to jump 10 centimeters.
7) Art design. The Uncharted series takes a picturesque approach with lots of epic moments and amazing views. Gears of War takes a dull alien scenery approach and lacks for instance arty death scenes and amazing lighting as seen in the Killzone or God of War series on the PS3. In the Uncharted series there also interesting females (also in terms of character design/development), Gears of War 3 looks to take the shemale approach instead.
8) Lastability. The Uncharted series include lots of hidden treasures, which make it more interesting to replay the campaign again. Also see my comments with regard to online play above.
9) Sense of humour. Gears of War tries to be funny like some aliens crushing some tiny mammels under their boots or boomers saying boom like a 3 year old when they fire upon you with a rocket lauchner. The Uncharted series takes much more clever and well thought "Indy" kind of approach IMO adults (and for example females around watching) will find a far more enjoyable approach.
10) Killing the bad guys. In Gears of War many enemies could only be killed by some remote satelite laser. You can't hurt those bigger guys in any other way. IMO this was used too often. In the Uncharted series you actually have to make plans when in cover. For example, first I am going to sneak kill that guy from behind, get his guy and hide behind the table and wait for the second guy when he comes examining. Then I am going to blow up the fuel cannister to kill the 2 guys there and then.. You get the point. Not nearly as much the case in Gears of War.
I could go on and on, but it's already a 10 - 0 victory for Uncharted! IMO the Uncharted series is way beyond Gears of War's league!
IMO Gears of War voters give Uncharted a try!