Ming Thein has completed his long term review of the Canon EOS 5DS R. It's a great review and very different than most of the reviews you see out there for a camera body. He compares the EOS 5DS R directly to Nikon's D810 and preferred a lot of things in Canon's high megapixel offering.
From Ming Thein
It is clear to me that neither Nikon nor Canon can claim to be decidedly superior at the moment, each with their own strengths and weaknesses – much like their equivalent lens lineups. Canon lacks Nikon’s new f1.8G primes, but Nikon lacks all of the special purpose stuff and some of the zooms – there is no 17 TSE, for starters. There’s no question that if I were a Canon shooter, I’d be very happy at the moment: you have a tool that’s miles ahead of anything else in the system, can hold its own against the competition most of the time, and can pull ahead if the conditions are right. Read the full review