Check out this video from Kai at Digital Rev as he does a hands-on comparison between the 50MP Canon 5Ds R and the 42MP Sony A7R II. This review is even a little more superficial than others he has done, but you get a clear sense of his first impression.
Personally, I think the 5Ds R and A7R II are mostly going to be finding their way into different kinds of photographers’ camera bags. With the A7R II being a more popular pick for a wider variety of images, while the 5Ds R is a little more niche for landscape and studio work where low sensitivity and high resolution are the primary concerns.
Have you considered these two cameras as competing options? Did you buy one over the other?