Hi there!
Years ago, when the hd650 still was sennheisers top headphone I compared the hd600 to the hd650. From memory the HD600 is somewhat leaner in presentation because is has less bass. But also the hd650 is a trifle better in resolving details. From what I remember the 650 had a "blacker background" (trifle less distortion I think) and slightly more realistic display of instruments (wood sounding like wood and brass metallic).
That slight advantage combined with a stronger bass for pop made that I choose the 650.
Whichever one you choose, remember that these headphones really need good amplification. You will hear everything. If the amp is not strong enough they sound bland, boring. If the quality of tge amp or source replay is not good enough it'll sound like a cheesgrater in your ears.
However if you get things right they are heavenly and addictive. Listened to them for many hours on end, iften forgetting time. It's as if your completely enveloped by the music, as if you are there and there's nothing in between.
Even after years I still rate them highly.