It's video is almost 100% exactly the same as the 5d3. With a bit more aliasing.
That's what I found after analyzing the frames. The 1DX has less blurring on the horizontal axis only ("anisotropic antialiasing"), leading to increased aliasing but not really much increase in actual detail/information. Sharpening both 5D3 and 1DX in post- the 1DX could not be sharpened as much (and required more sharpening on the vertical axis). In any case, the 5D3 post sharpened looked better than the 1DX (also post sharpened as noted): less artifacts. Cranking up the bitrate on the 5D3 will really help improve image quality, as post sharpening's limit is due to compression artifacts.