Kinda like both setup. I use AKG K7XX at home straight out of my DAP. It is quite flexible, got a longer cable so I can listen while riding my stationary bike, easy to drive straight out of my DAP. Also use it when I play game occasionally, sounds awesome, much better than my previous Logitech headphones, but don't have Mic though, so if have to use Mic I would still use my Logitech headphone.
The HD6XX has a shorter cable, I use it at my office with the Schitt Magni 3 amp. Both the HD6XX and Magni 3 are kinda new, so it is like both are going through burn-in slowly. I don't intentionally do the long hour burn in, just burn-in by listening 1-2 hours a day. Initially I am not impressed by HD6XX, but it is gradually sounding better now.
The HD6XX seems easier to use a balanced cable (cable attach points at both sides), and I seemed to recall the Sennheiser headphones are usually better with balanced setup.
To me the AKG K7XX is slightly more comfortable. Both are quite light. The HD6XX gives a bit clamping feel, but not too bad.