Frame Rate (12 vs 6) Buffer size, Significantly (apparently, I haven't dropped it in water to find out !) better weather sealing, better batteries are the big obvious differences.
IQ is good on both, I feel the 1dx has the edge with the larger pixels, but there is very little in in it.
Note: I've only done cursory tests at this point, next couple weeks in Iceland will be the telling factor.
Regards
Mark