If you shoot in artificial light, the flicker mode on the 1DX Mark II may be worth an upgrade from 1DX.
Better viewfinder information and improved focus system in the 1DX Mark II is nice too.
But in general, at least as long as you stay away from video, the differences between 1DX and 1DX Mark II aren't such that I would have upgraded, if it wasn't for the accident my 1DX had.