Nikon D3X has a 25.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed processor. On the other hand, Nikon D3S has a 12.0MP Full frame (36 x 23.9 mm ) sized CMOS sensor and features Expeed processor. Nikon D3X's sensor provides 13MP more than Nikon D3S'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 D3X and D3S sensor size comparison.
Nikon D3X and Nikon D3S have almost the same sensor size so none of them has any significant advantage over other in this area.