Uncharted 2, without a doubt.
Let's look at sales. Ignoring the fact that we are comparing a stealth game to a platformer/TPS.
Uncharted: 2.34 million
Double Agent (360): 1.14 million.
Userbases?
PS3 when Uncharted released: 7.83 million
360 when Double Agent released: 4.87 million
So really, not that far apart.
They have roughly the same metascore, so they received roughly equal reviews.
Splinter Cell is a series on a sales decline. And a quality decline. Meanwhile, Uncharted 2 looks to be far better than the original, and is launching on a far larger userbase without the staggered launch. And it's not a new IP anymore. Bear that in mind. Double Agent came off a 3 million seller (on Xbox alone) and did 1.14 million. Uncharted, a new IP, sold 2.34 million. More than both Jak 3 and Jak II.
I would expect around 1.7 million for Double Agent, and 3.5 million for Uncharted 2.
(Former) Lead Moderator and (Eternal) VGC Detective 
