So this might be comparing organic apple vs. regular apple. And bear in mind that this is merely one aspect being compared, which is high ISO.
For those curious as to how the two cameras stack up against one another from iso 6400 and up, I'm providing a small sample of comparison photos. iso 6400, 12800, 25600 & 51200 were compared.
(Note that these are screenshots of my LR4. They were captured at 1920x1200 and then uploaded to my smugmug account. Obviously staying within the resolution rule here at POTN, the images below are then resized from 1920 wide down to 1024 pixels wide.)
On the 5D3 images, I had to reduce the exposure by ~1/3 of a stop. It seems that (at least for my copies), the sensitivity of the sensor for the 1DX is ~1/3 stop lower than the 5D3. I also noticed that the red channel was also brighter, but also noisier for the 5D3 as well.
Full image comparison: (1DX left, 5D3 right)
iso6400 crop: 1/800s, f/4.0 (5D3 also reduced exposure by -0.3 in LR)
iso 12800 crop: 1/1600s, f/4.0 (5D3 also reduced exposure by -0.3 in LR)
iso 25600 crop: 1/3200s, f/4.0 (5D3 also reduced exposure by -0.3 in LR)
iso 51200 crop: 1/6400s, f/4.0 (5D3 also reduced exposure by -0.3 in LR)