The only reason beyond aesthetics to get an HD650 over a HD6XX is warranty. Drop refuses ALL SERVICE past the warranty date. Even if you offer to pay for any repairs needed, they will refuse because they offer no such service in any capacity. Failures within the warranty period result in a complete swap of the device (you get another pair). Outside of warranty, you're straight out of luck with Drop.
Which makes me feel bad for all the unfortunate folks willing to buy an HD8XX (no balanced cable, and no warranty at all after the period, which is insane on a headphone of it's cost).