Its a bit of an apples to oranges comparison due to the outside lighting, but the first thing I noticed was that the D750 images was a bit brighter. If you have the raw file, it would be interesting to see if the detail that is not present in the jpeg is there. I suspect so unless the highlights were blown out. What makes me tend to like or hate a sensor more is how well I can recover detail from and ETTR image and how well I can recover any details from an underexposed image without too much grain and muddiness. Newer and larger sensors often tend to be more forgiving in these areas and in high ISO images.
You are 100% correct regarding the brightness of the D750 image. It has a tendency to overexpose by 1 or 2 stops. That was an image I shot back in December. Since then, I have made adjustments to the D750 to get better exposure.
Hopefully some time next weekend I try to do another experiment using 4 cameras, (similar lighting, lenses and settings) to have a better set of images to compare. I'll shoot in both RAW and jpeg to eliminate any file conversion factors (although I always use Adobe Camera RAW to convert files; without making any adjustments).