Different animals. Balanced HD650 will certainly have a faster impulse response than a SE HD650, and bass will be a lot tighter. Detail will be better, imaging will be sharper, and things will be more transparent in general. Although you're not guaranteed to have really good bass, it's possible if you voice your system that way, and you really can get some of the deepest, hardest-hitting bass around.
The comparable 'stat rig would be an O2 with an amp that can drive it right. This will have a faster impulse response still, more transparency, more accurate tone, and more air and space between instruments. But, it won't have quite the impact or harmonic richness, and it won't have the (artificially) massive soundstage that you'd get out of a balanced HD650, though the imaging will be even better.
What I'd do would depend on the music I listened to, and of course the amount of money I had to play with. I don't find the O2 to be a very good electronica headphone; it doesn't have the artificially large soundstage or the massively impactful bass, at least compared to the balanced HD650. On the other hand, for acoustic music, and for rock, strangely enough, the O2 is supreme. It really sounds far more like the real instruments than a good system trying to reproduce said instruments. So, if ultimate realism is a factor, then the O2 would get my vote, but if a system with a bit more coloration and a somewhat more forgiving nature was in the works, I'd go with a balanced HD650.