I much prefer the D810 for high ISO photography, it has way more detail in the image than the D3S. I used both for northern lights one winter, and the D810 is superior. The D3S may have slightly less noise than the 810, but also far less detail. When I resample the 810 files to 12 MP the noise decreases, but detail is kept. If I resample the D3S files to 810 size, noise increases greatly, and details in the photo is just mush. So I sold the D3S, I had no use for it as I don't need the speed.
When comparing noise, it is important to resample the photos to the same size. And also look at the details in the photo, not just the noise (grain structure).
(And the grain structure from the D810 is just beautiful, flim-like and random.)