From what i've seen/read, the 1Dx is considered soft and lacks detail like the 5d3, with slightly better low light performance. The s35 crop mode on the 1D-C is much better, and shows what they could have done on the 5D3 and the 1DX.
It's too bad they didn't put an APS-C video mode in the 5D3 because:
1. more reach can help for wildlife
2.a. it would be closer scale to the natural AA filter and might not need to be quiet as softened up
2.b. perhaps the single digic would even be fast enough to simply reach that whole chunk directly and downscale to provide really high quality
3. they could even perhaps have fit in an even high mag mode where they read out a 2k block directly and process it
Anyway I've heard that the 1DX tends to have a bit more moire in it's good mode, but to offer noticeably better micro-contrast/crisper detail. It will never have ML made for it, at least no by the ML group, and considering all that Canon left out of it that is a shame and makes it less useful.