I just got my Atomos Ninja 2 today and did some test with the D750.
1080p 23.976 with the internal codec vs Ninja 2 Prores HQ
I also did a test with 1080p 60fps internal codec vs Ninja 2 720p 60fps Prores HQ with up-scaling and some light un-sharp mask in CS6 Premiere.
both at ISO 800 and with the Flat profile.
For 1080p 23.976 the Atomos is not a huge leap in video sharpness,( since the codec is doing a very impressive job) and its hard to tell the difference, especially compressed for web. But I am noticing some better color reproduction and saturation since its 422 compared to the 420. It slightly sharper with the ProRess, less blocky, and noise looks more organic and natural. For the 60fps I was surprised that the Prores HQ at 720p could be up-scaled some sharpness added and almost match the internal 1080p 60fps codec (and look far better than the stock 1080p on the 5D mark iii.).
I used to have the 5D mark iii and shot with the magic lantern RAW. Though I love RAW, it was so much of a pain to use the hack that I don't regret switching to the D750. Photography and video is by far sharper, cleaner, much more dynamic range, and very accurate color science ( which is a huge advantage going back to 8bit video from RAW, I barely need to color correct as long as I get the exposure and white balance correct in camera). And before Andrew's review about this camera I would of laughed at anyone who said you should shoot video on a Nikon.
Now if this camera could just get some love like the Canon DSLR have had from magic lantern, or Nikon really make some serious firmware updates for better video features, this thing could out beat the current competition, except for the low light capabilities of the Sony A7S (the D750as good or little less than the 5D Mark iii). Also not to mention for photography it has 12bit compressed RAW option, just imagine if Nikon used that for RAW video.