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 One could argue that the Uncharted series' plots of "Find the treasure-discover the truth about the treasure-thwart the villain who's also looking for said treasure" is forgetable, too..... Actually after completing Gears of War 1, I forgot the plot, actually even before that Sorta like how Uncharted forgot its plot about the (I'm not even gonna try to spell it) Stone--that wasn't even a stone despite us being told before hand that it was....... 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 I won't go into that, again. Let me suggest you play through Uncharted 1 (and even Uncharted 2) again. I did recently (despite the fact that my PS3 sig won't update for some reason). Technology advances fast. Nostalgia stays the same. The games look good but 360 owners have nothing to be jealous of.
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...(This is a knock against Gears of War or your take on Xbox Live?)
6) Gameplay mechanics. Drake is a lot more flexible with regard to movement than Marcus. Drake can roll dive (so can the guys in GoW), hang from the edge of a building while aiming and shooting . You can perform various pushes and kicks (But can he wield a chainsaw bayonette, take an enemy hostage, or give them a beatdown when they're on the ground?). Marcus can unrealistically crouch run (Like an imaginary rodie at a concert or a soldier looking for cover?)(I'm a physiotherapist (I'm an ex-soldier/ex-cop/current armed security officer that trains every couple of months in military techniques including building clearing, moving and shooting, night firing, one hand weapon clearing, moving from cover to cover, etc.)) but is unable to jump 10 centimeters (Video games =/= reality. If they did, a guy who just got shot in the stomach probably wouldn't be able to climb mountains and take on an entire army......)
7) Art design. The Uncharted series takes a picturesque approach with lots of epic moments and amazing views. Gears of War takes a dull (Your opinion but the main theme of Gears of War is "Destroyed Beauty" and the games take place in Victorian/European influenced environmens....as well as underground, inside of alien worms (?) in the air, in icy environments, and ruined cities. It may be an alien world but it's based on earthly architecture.) 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 (Gears 3 should fix this. It was explained that in the earlier games, women's role was to keep the human species alive. In Gears 3, almost all hope is lost. Women have to fight, too. And while a little cliche, I found the part about Dominic's wife to be very powerful. My opinion, of course. And in terms of character development, seeing Carmine go from a wimp to a badass (before dying) and seeing Tai go from badass to broken man in Gears 2?) (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 (Like looking for dogtags and relics in GoW1 & 2? There's probably as much to find in GoW2 as there was in Uncharted 2. Plus, Campaingn co-op split screen and online co-op if you're into that sort of thin). Also see my comments with regard to online play above.
9) Sense of humour (Like when the guy turns of the lights in Gears of War, knowing that the Kryll would attack in the dark--before turning the lights back on and saying "I'm just fuckin' with ya!" or the back and forths between Baird and Cole? Or Cole's "Rap"?) . 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 (That only happens with two Bersekers and one Corpser. In Uncharted 2, there was that guy at the end of the train level who was pretty close to invincible or the boss who could only be killed by blowing up amber close to him. I'm sure there was more but I only played through either campaign once instead of multiple times on multiple difficulties with multiple players like I did on Gears 2). 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 (and I could, too), 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! (And I really wish you would give GoW2 a try, too)