I just got my Denon D400 and man, I am pretty much happy. I wanted D600s, but no one wanted to give their cans up, so I went with the lower model. Here are the Pros and Cons (with D600 compare):
Pros (8): regular adjustable headband (D600s fit my head almost perfectly, just a little loose on the smallest setting); snug, but not too snug earpads (D600 were too roomy around-ear and not deep enough inside); smaller earcups for less clunky fit (D600 earcup sides stayed parallel; sux for uneven sides); Earpads contoured head like a pillow after being used to ear-and-head fitting (D600 earpads fitted earcups more than your head, making two earcup bowls on one bowl headband fit); one small cable connector (D600 cable had large connectors on both sides); deeper bass with semi-lush mids and part-sparkly highs IN PASSIVE MODE (my HE-4 makes up for D600 spacious sound improvement); smaller frame (still not foldable, like D600); a great value (D600 is only a better value to the D7100)
Cons (5): earcup plate knobs for built-in amplifier and remote (D600 looked less ridiculous by ears facing someone, but headband looked more ridiculous); earpad and headband stitching is less thick (D600 has luxury-car-like leather seaming); more frame flexibility and knobs make build feel kind-of cheap (D600 plastic and design is more durable, just not for you); less spacious sound (D600 huge earcups help here, just not for fitting); built-in amplifier works best at lower volumes (D600 scales well with powerful amps, sold seperately)
Amazon.uk has the D401, which supposedly improves the built-in amp, for rather cheap, so I highly recommend it. Just get used to headband adjustment with earcup rotating for a few days before your flight to get used to fitting your head. Also, built-in amping leaks a good amount of sound, so hear how it works from outside the earcups, so that you can be considerate to others around you. Lastly, burn-in the drivers with pink noise and some songs on repeat for at least 25 hours total, so that your trip will have smoother sound.