I'm biased here. The following is a tilted rambling overemphasizing the etys strengths and exaggerating the HD600s many weaknesses
. When I started down the same path you're treading five months ago, a wise member told me that among the creme of the crop there are no clear winners, just personal preferences. Regardless of which headphone you get, you will be happy. The only question is which one will make you HAPPIER. I believe that the Etys will.
For active use (which means you're not sitting around slacking) both headphones suck. I am not going to be walking around with a dang HD600, required amp, and audio player (especially with the clou cables). I'm not going to walk around with an ety either due to microphonic issues or sound isolation.
Even ignoring those issues, do I really care about the detail of my headphones when I'm moving around? If I'm moving around, I use my KSC 35s... the best $15 investmestments I've ever made.
My true headphone quest began when I was searching for a stationary listening audiophile quality headphones. I searched the boards for a while before I made my purchase of the Ety4P. I found only one dissenting opinion (acidtrip guy) on their impeccable sound quality. With the HD600s there were far too many assorted opinions (people said the $100 clou cable was required, the HD600 was too dark, the HD600 is extremely dependent on the amp, it sucks compared to CD3000). While I was happy with the nirvana of the Etys, I eventually purchased the HD600s with red clou (their ultimate form!). The sound was different and enjoyable, but didn't justify the cost. The HD600 costs close to $240 (cheapest), clou cost $100 and require an amp. The Ety 4P costs $270.
When I had the HD600 with red clou, I didn't think they blew away the etys (using a cosmic headphone amp). Durability wise, I have no doubt that my Etys with rubber tips/cable could survive a 30 foot drop (not that I would try anything like that), while I doubt that the HD600 could survive crammed in a backback (which I'd have to stuff an amp in as well
). This is mostly due to mass and design of course.
When I want to immerse myself in sound, I use my etys. I can sit down in my special listening chair and close my eyes. I can also sit on a bench in a crowded street and have a similar experience. The Ety4P would work great with the lousy headphone out of my minidisc player, and with the 4P-4S adapter it would be ecstacy with the cosmic amp. I don't have those options with the HD600.
I think you should grab the Etys because they are more smaller, cheaper, a consensus of people who have used both would say they sound better (at least the last time I checked), and they give you more options (4P, or + adapter = 4S).
Of course, this is all my opinion. Good luck!