What is interesting to me is that the rumored 4K 8 bit 422 MJPEG comes in at about the same bitrate as 4K ProRes 422 (500 mbps vs ~489mbps). Both are intraframe codecs.
I'm not familiar with MJPEG, so I really don't know if it's similar to ProRes or DNxHD from an editing standpoint, but it sounds like it could be. Or possibly a clean 422 signal could be sent over HDMI to an external recorder where ProRes 422 or DNxHD could be recorded. That would be a welcome change for a lot of people I think (assuming the codec is an improvement over what is in the 5DIII).
It would be nice to see C-Log included as well, but since it's not available in the 1DxII I suspect it won't be in the 5DIV either.
Good question; I don't know (anything, frankly) about the internals of ProRes, so had a quick look at Wikipedia
I guess it'll come down to efficiency. Wiki says it's DCT based, so that's the same as JPEG/MJPEG (JPEG/MPEG 2000 use a wavelet algorithm instead).
On paper then there might be little difference; though ProRes 422 is claimed to have a 10-bit sample depth, and I'm assuming MJPEG is only 8; so the former might give more detail and better results with heavy grading.