Aloha,
sometimes we forget that these cams are firstly made for photography and video came on top later as a kind of "feature". That there is still a huge imbalance we can see here http://gizmodo.com/5936270/the-canon-1dx-makes-one-hell-of-a-5k-movie-camera Nevertheless it seems that Nikon and Canon finally presented "usable" cams this year. I never understood why a Mark II hadn't had a headphone jack from the beginning. Isn't that something absolutely necessary???
The price difference is in my opinion not only the video quality but also for stills - beside the fact that with a 1D X you can shoot outside wherever & whenever & at nearly all weather conditions without any fear. This one is build for sport and journalism. A Mark III seems to be more a "backup" I think.
But can I ask you a question, beside all this video thing: do you think the Mark III will be "sharper" and with more "dynamic range" when using same lense and settings than the 1D X? I am not an crop fan so I shoot nearly exactly that what I want to shoot - and not later at PS or LR. Has anyone some experiences to this point?