I do like the HD-650 more than the K7XX, it has one of the best over all sound signatures for almost all genres of music IMO. It also will scale with better equipment and its quite comfortable. The K7XX is good but its not a headphone that I will reach for often and I've found that it can be picky with amps. Some love the K7XX over the HD-650 for the better sound stage and bass that is a little cleaner while others like the HD-650 for its forgiving nature of poor or sub-par recording. Only you can really decide which is the better headphone for you.
I can and do listen to almost all genres of music with the HD-650 except for classical and large orchestra pieces, this is where the average to less than average sound stage of the HD-650 is at a disadvantage. I usually listen to jazz, R&B, blues, pop, Kpop, Jpop, and rock on the HD-650 but depending on which amp I have on I might use one of my other headphones.
When I first get a headphone I will normally play a set list of songs which has all the genres I normally listen to and it helps me to understand the strengths and weaknesses of that headphone and I will usually do the list of songs a few times with different DACs and amps to see what sounds good, great, or so-so, but this is my preferences only.