Yes, the Canon 5DS R beats out the Nikon D810 visibly. As for the Canon 5D Mark III, it has seen its day and will keep many shooters happy, but mine is for sale.
I have yet to formally evaluate dynamic range and color with the Canon 5DS/R but impressions are already firm: it’s no Nikon D810 in those areas (and ACR has issues with 5DS files).
If you want the best sharpness today in a DSLR, go with the 5DS R, which has no anti-aliasing filter (Zeiss Otus and certain other quality lenses advised). The 5DS with its anti-aliasing filter lacks the same micro contrast (subtle the plain to see at all times), and I don’t recommend it for peak sharpness; see the comparisons in my review such as the 4-way comparison.
As I see it, lovely image quality gains are possible with a 100+ megapixel DSLR (with the right lenses). Even against a monochrome sensor, oversampling works. And the naive assumption about megapixels being all about sharpness is a simplistic viewpoint that ignores all the other benefits. All that is needed is advances in sensors to make 100 megapixels a reality (with the quality of today’s 36-50 MP sensors). Sony is already on that track with its 42-megapixels sensor in the A7R II, and the RX10/RX100 sensor density is 148 megapixels, so it’s only a matter of time to scale that up and improve upon it for full frame.