This is not easy, comparing the HE-500 to the HD600. The HD600 is more sensitive and easier to drive as long as your amp can deliver enough voltage for 300 Ohm cans. So when flipping the A/B switch I also have to twiddle the volume knob on the amp to get them as close as possible and that is not an exact process. What I can tell, so far is that the biggest difference is that the HE-500 has bit more sparkle in the treble which to me is a slight exaggeration but not enough to make silabance grating or to be fatiguing, The HD600 has a drop less extended bass and perhaps a touch of mid to upper bass hump. They sound very similar where the most obvious difference is the HE-500 treble sparkle (not preferred) and the HE-500 better bass extension and slightly better definition of bass notes (preferred). Both deliver nice impact to drums. The HD600 goes for $400 and the HE-500 goes for $700.
If I had to pick one, I'd get the HD600 due to the price difference. But I have both so I don't have to pick just one, hehehe. I prefer these two cans to either the Sennheiser HD650 or the HD700. I think the HD800's are even better, but not by a large margin and they do cost way more, typically $1500, ouch, that old story of diminshing returns.
So for tonight the A/B switch is taking a rest so that I can go back to enjoying listening to music. Time to flip a coin and pick the cans. Tomorrow I can go back to being analytical with different music and try my patience some more.