While I no longer own either of these cameras, there was a short time where I owned both! That made for at least one fun night out under the stars where I decided to compare the high ISO performance of each in wide field astrophotography situations. I sat on those images for a long while, but have finally gotten around to looking at them and comparing them side by side.
Being the newer camera and also the lower resolution camera, the hypothesis was that the high ISO performance of the D750 would be better despite the higher price of the D800E. The results bear that out, with my unscientific objective analysis placing that difference at about 1 stop of high ISO performance improvement in the D750. That is to say, the D750 at ISO6400 shows about the same amount of noise as the D800E at ISO3200. Of course, this can be mitigated to some extent by downrezzing the D800E to the same 24MP that the D750 shoots at natively.
While I don’t have any images of it, I also noticed an improvement in the D750 in handling thermal noise (also known as purple banding). You often see that in purple noise emanating from one corner of the image when shooting long exposure, high ISO images on a warm night. It can be reduced or even eliminated by either shooting with long exposure noise reduction on (which doubles your exposure time while the camera shoots a dark frame to subtract the noise) or by shooting dark frames manually and subtracting that noise in post. I will be writing an article further detailing this soon, but for the sake of this article let’s just say it is less of an issue on the D750 than on the D800E.