Nikon S9100 has a 12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Expeed C2 processor. On the other hand, Nikon S9500 has a 18.0MP 1/2.3-inch (6.16 x 4.62 mm ) sized BSI-CMOS sensor .
Nikon S9500's sensor provides 6MP more than Nikon S9100'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 S9100 and S9500 sensor size comparison.
Nikon S9100 and Nikon S9500 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.