Sony A7S has a 12.0MP Full frame (35.8 x 23.9 mm ) sized CMOS sensor and features Bionz X processor. On the other hand, Nikon D800 has a 36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 3 processor. Nikon D800's sensor provides 24MP more than Sony A7S'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 A7S and D800 sensor size comparison.
Sony A7S and Nikon D800 have almost the same sensor size so none of them has any significant advantage over other in this area.