Both Canon 5DS and Canon 5DS R have Full frame sized 51.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another difference between these two cameras is that Canon 5DS R's sensor lacks anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the 5DS and 5DS R sensor size comparison.
Canon 5DS and Canon 5DS R sensors have exactly the same size so none of them has any advantage over other in this area.