The AD900x and HD600 should be quite different sounding.
The HD600 should be darker, warmer, bassier, etc. while the AD900x should be lighter and arier with a bigger sounding soundstage. The HD600 probably has more emphasis on male vocals while the AD900x has more female vocal emphasis. Movies might be more enjoyable on the HD600 if it has a thicker sound with more bass. Comfort is subjective, but both should be pretty good. The HD600 will have more clamping pressure, while the AD900x will be lighter but with the "3D wing" headband system, which some people seem to dislike.
Best option of course would be to try them both. If you can't do that then read reviews of both of them and see which ones sounds like it would be a better fit for what you want.
The 3D wing is mad crazy to get used to... took me a week just to learn how to get that dmn thing to fit on my head... but once I did, man what a great idea. they are ultra light on my head [and they are very heavy]
http://hifilittleriver.blogspot.com/ here's a link to me blog <3. Posted some nice pics of the w1000x :D
Edited by Mshenay - 8/3/13 at 11:46am