Well is the QuietComfort quite comfort (pun intended)
The QuietComfort is a closed-back noise cancelling headphone and the AD900X is an open back one.
Even if we're assuming they have equal sound quality from the drivers, they're still quite different.
For example if you're in a quiet room, the AD900X probably beats the QuietComfort, because open-back designs doesn't block sound = no echoing = better sense of "room"
But the AD900X doesn't isolate any sound, so if you're on board a noisy jet, the QuiteComfort would probably be better.