I wouldn't worry about a learning curve between a 5D MK III and 1Dx, the controls are fairly similar.
The choice is based on how much tolerance you have to noise, and if your lenses do a good job for you at f/2.8.
Here is a shot with my 5D MK III at ISO 25600, f/2.8 and 1/80 sec. Of course its noisy, and loses detail from NR as well as the noise. Its just a matter of what you can stand. BTW, it will still print well at 8 X 10.
At ISO 12800, f/2 1/50 sec with my 5d3 and 135mm L. Once again, resolution is lost due to noise, but its quite usable. Colors are off a little, the auto setting in Lightroom makes them a little warm.