I'll start off by saying that will be a tough one for me.
The Edition 8 as a portable is probably, unquestionably actually, the best I have ever seen or used for laying on your head nicely and comfortably. It kind of hugs your head and with the way the cups surround your ears it's just so sleek and low profile. Doesn't move a bit - the clamp is perfect. That said however sound wise is quite different than the TH600. I would say the clarity of the Ed8 is extremely good as it is the TH600, but where the big difference is to me is in the bass response arena. The edition 8 is no slouch, but the 600 is just a beast. I mean it will handle anything you throw at it. Out of my vintage Pioneer Spec stack it almost seems like you couldn't distort it. The word glorious comes to mind when describing the TH600s bass response.
So honestly if I were to choose one and could only have one I'd choose the TH600. That's saying something. Because I love my edition 8. The more I listen to my Edition 8 as my portable set up on my arm for doing whatever walking running it just is so good. The TH600 is not what I would call a good portable solution compared to the edition 8 but that's the only downside in this battle.
Love them both......