Uncharted 4 will sell more, probably won't be that close.

There is a lot of hype around U4, helped a bit by the fact that it is the last in the series.

Microsoft's other big franchises are in significant sales decline, I would expect Gears to be the same.

We haven't seen anything about gears 4 and you are basing its multiplayer as something special. That studio also never made anything and had several internal projects which got cancelled before working on gears 4. You are jumping to conclusion.
As for uncharted 4 it's made by a studio that is a legend in the industry.
Sales, graphics, metacritic (uncharted 4) >>>>> gears 4.

Uncharted 4 will crush Gears for in first week sales. Its so obvious. The Xbox One doesn't have the bandwidth of install base to take even touch Uncharted. You will need to circumvent reality in order to come up with a realistic reason why Gears beats Uncharted first week.

Definitely Uncharted 4.
Gears of War 4 will have no chance against Uncharted 4 (sales + quality).

Gears has always been forgetable for me. Uncharted is a different story. My money is on Uncharted 4.

As people have stated there is way to much going for UC4 than GOW4. Install base, last of the series, ND track record. Release Date, you name it. I personally did not like UC3 all that much but will be getting UC4 day one. The game looks fun and it releasing at the perfect time when I will have caught up with my back log.

I feel exactly like that.Except Gears is the pizza. All flash, no substance. Uncharted 2and 3 are games I still play nowadays. 

Anyway, the answer is obviously U4. 

first week? probably gears.
lifetime uncharted for sure

Won't even be close. Uncharted 4 is going to sell way better.

Easy, Uncharted4.