I think that you may want to first ask yourself what the application would be for the headphones. If it is for mastering, you would want something flat; if it is for listening, something with a little more euphonic coloration may be good. It's all personal preference.
The headband is falling apart on my ATH-M50s so I am also in the market for a replacement. I really, really like those headphones, but am searching to see what else is out there. I have read good reviews of Sennheiser Momentum headphones, and may take a look at the Sennheiser HD600 as well. I don't master too many recordings anymore so the Sennheisers may be a good choice. The Momentum is closed-back and the HD600 is open-back, so that is another variable as well. Finally, I could run the Momentum straight out of an iPod, while I would need an amplifier for the HD600. All things to consider when looking at headphones.
My main concern now is whether the double and triple price difference of the Momentum and HD600 over the M50 is worth it.