HUGE.
AD700 is known for their openness and big stage, but it has no bass. If you play any type of FPS, bass is your enemy. HD650 soundstage is good, but not Audio Technica good. HD650s have a lot more of everything else, bass, mids, smoothness, and it's a great headphone if you like classical / jazz / acoustic stuff.
I personally have HD650 and A500 (which is more or less in that same family). I prefer A500 for gaming, but if I listen to music it's HD650.