Both Canon 5DS R and Canon 5DS 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 R and 5DS sensor size comparison.
Canon 5DS R and Canon 5DS sensors have exactly the same size so none of them has any advantage over other in this area.