The 650 is newer and the pads are kind of stiff. My 600s are about 8 years old and I replaced the pads, out of the gate the pads we much more pillowy. I use the 600s on my non-DAW only.
I prefer the 650s to the 598s in how they are both constructed and feel on my ears. Since both of mine were profiled by Sonarworks, the delivered sound isn't much different unless I just play them without Sonarworks enabled.
FWIW I keep a spare iLok on hand and I pay the $30 per year insurance so that if my iLok goes bad, is broken, lost or stolen, I can send in my spare and all my licenses will be transferred to the new iLok with overnight shipping both ways.