It really comes down to the music styles you listen to and what you are pairing it with. If you listen to vocal music: i.e. jazz, folk, soft rock, adult contemporary, soul, etc. it would be the 650 for me all day and night. If I listened to more electronic music, pop or faster music with more dynamicism, like hard rock or metal, then I would consider the 600/660S (I am convinced these are the same cans) unless my DAC amp combo was clinical, then I would want the 650's to balance it and give me a bit of the warmth and weight back to my music. The good news here is you can't lose with any model. For me I would choose:
HD650
HD660S
HD600/6XX

In that order....
I would be pairing it with a warm DAC and a clinical HPA in my case.