I'm pretty confident Gears of War 3 will outsell Uncharted 3 overall. Gears 1 and 2 suffered from being a bit rushed, and Gears 3 gets the longest development and testing time. So it's going to be noticably more polished then the other two, and both the sales of Gears 1 and 2, and strong pre-orders imply that Gears 3 will at least break 6 million lifetime. Uncharted 3 might be the biggest Uncharted in the franchise, but Uncharted 2 is still 1.7 million sales behind Gears, and it's going to be competing for attention with Resistance as well. So Uncharted 3 might break 5 million sales, but I really doubt it will pass Gears.

As for Week 1 sales, no PS3 exclusives have yet to record massive week 1 sales. Uncharted 2 only did 850k week 1, and Uncharted 1 did a measly 133k. Gears of War 2 did close to 2 million week 1, and Gears 1 did 910k. So even the debut title in 2006 had more sales (lifetime, and week 1) then Uncharted 2, arguably the best rated PS3 exclusive (alongside MGS4).

Personally I think Gears 3 has a shot for 7 million, 8 if CoD really drops the ball this year (Sledgehammer is developing, not Treyarch, or Infinity Ward and Activision has states they expect less volume). Uncharted 3 also has a shot for 5 million IMO, and it might crawl to 6 million if CoD drops the ball.

P.S. Judging from the relativly balanced votes for both games, and the inflated PS3 favoring audience on VGC, I think most people realize Gears will be bigger. I say inflated PS3 favoring audience based off the Most Wanted section when multiplatform games on PS3 get as much as 10x the votes of equivalent 360 titles or more.