Im assuming you want to know what the difference between 60fps at 720p and 30 fps at 1080p looks like?
If so check this out.
http://www.youtube.com/watch?v=sjk1SXPJR2Y
You can also play a game on the 360 then play on your pc at same resolution and see the difference. Games are a bit slower at 30fps.
Obviously 60fps is preferred. But you will have to sacrifice settings to go from 720p to 1080p and keep 60 fps. As you are doubling the pixels rendered.