Both Mass Effect 2 and Uncharted 2 are excellent games - it really comes down to of you prefer shooters or RPGs. If you haven't played ME1 definitely go with UC2. Personally, I've spent more time playing UC2 than ME2 because of the multiplayer, but with UC3 coming out soon that might not be a good investment.
I'd say rent UC2 for the single player and buy ME2.
It just looks like Tomb Raider with a male lead. But...different tastes for different people I suppose.
Not even close.