Nikon D810A has a 36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features EXPEED 4 processor. On the other hand, Canon 5DS has a 51.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Dual DIGIC 6 processor. Canon 5DS's sensor provides 15MP more than Nikon D810A's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the D810A and 5DS sensor size comparison.
Nikon D810A and Canon 5DS have almost the same sensor size so none of them has any significant advantage over other in this area.