The difference is not that huge between 900p and 1080p in current gen games, the low texture resolution and heavy (shoddy) AA methods hide a lot of potential detail. You can still see a real difference in a simple comparision.
Crysis 2 on Ultra, 3 screen grabs.
1280x720 resampled to 1920x1080 (right click to view)
1600x900 resampled to 1920x1080
1920x1080 (ignore the window frame, win 8 won't take screenshots in full screen mode)
Comparison of the 3 pictures, cropping out the sign on the fence
The difference is only going to get bigger with better texture resolution and more refined AA methods.