The 2 big juggernaut 2011 exclusives.
Which will sell more lifetime?
Uncharted 1 sold 3.5 million, and Uncharted 2 sold 4.42 million.
Gears of War 1 sold 6.13 million, and Gears of War 2 sold 6.14 million.
Uncharted 3 is definitely a big underdog compared to Gears of War 3. BUT Uncharted 3 at the time of its release will have (for a PS3 exclusive) unparalleled hype. I think Uncharted 3 will outdo Uncharted 2 by at least a million. And unlike other PS3 releases that don't usually do well in the US, Uncharted 3 will have the biggest sales support (likely biggest opening) from the US.
Gears of War 2 despite bigger advertising drive/hype/ didn't outsell the 1st Gears of War until a month ago. I wouldn't be surprised if Gears of War don't quite outsell the previous Gears of War.