The auto focus in the 1DX is much faster. The shutter is super fast. I actually like it more than the 1D Mark IV. The 5D Mark III seems to struggle a bit in lower light or objects with little contrast, but the 1D X has been hitting those.
The downside is that the two do not expose and meter the same. The 1D X looks wrong. It isn't metering the same as the 5D Mark III and I actually think, compared to the 1D4 and 5D3, it is slightly underexposing. Auto ISO in manual mode gives different ISO values on the two cameras, the lower being on the 1D X and thus it does look a tad underexposed. More into that later.
I will next compare outdoor shots again with plenty of light of a church.
So far other than the 1D body and the faster shutter and burst rate, I really don't see much advantage over the 5D3. The ISO performance by my indoor examples are not significant enough. They are super close up through 10,000 or 12,800. The landscape shots really weren't different enough among the 1DX, 5D3, and 1D4 to really say any difference there, other than the pleasing extra detail present in the 1D X images.
I'll do two more tests. The normal shooting situation of a church, and then some action/sports. Those will really be the true tests, so stay tuned.