I don't think one is clearly better than the other. I've had both the 600 and 650 multiple times. The 600 has always won out for me. Not for any big reason, but I found the bass in the 650 to be a little too much for my liking. Which isn't saying much, because it's not really that much at all. Just in comparison to the 600, I think the 600's are overall more neutral sounding, which is just my style. I'm always blown away by how much detail they retrieve.
Anyways, at least to my ears, I didn't hear this massive difference between the two. I'll be the first to admit I'm not an expert/audiophile. I don't have the best hearing so take what I say with a grain of salt. I'm more of a casual audiophile. But to me, the two were very close in sound. The other thing that edged out the 600 for me is the build. I find the 600's design to be more comfortable. I know the 650's clamp loosens over time, but I just find the 600's headband + ear pieces to be even more comfortable when they loosen up too (so they are more comfortable out of the box, and after they break in).
You can't really go wrong either way though. Both are IMO the best headphones I've ever owned. I love them. I just ultimately liked the 600 slightly more, because of the more neutral sound. Even though they don't have a huge bass presence, you can hear the bass in all its glory (if that makes sense). They are just so amazing at getting all the details, its a real pleasure to listen to songs with them.
Edited by satanigatan - 1/4/14 at 7:23pm
I voted for "600 clearly better than"...but just because I prefer it slightly. I don't think it's drastically better, only slightly better. And only because of my own preferences. If the 600 didn't exist, I would gladly take up the 650 as my current end game set up.