I'm going to say the technically better experience is MGS4 (Uncharted had some texture pop-in and screen tearing half way through, especially in the jet ski section, in my experiences of 2 run throughs). I experienced no graphical issues with MGS4 (which is surprising given how good the game looks most of the time).
But to be honest, after playing Uncharted a second time, the locations (the level of detail when you're scaling the side of anything i.e cliffs) and character models make it much more impressive for me when it's engine is on stride (which is 98% of the time, minus the jet ski segment and the plot twist, which brought with it some pretty uninspiring indoor visuals).
So I'm going to say Uncharted (U2: AT looks amazing from what I've seen so far).