Sony S950 has a
10.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Nikon S9900 has a
16.0MP 1/2.3' (6.17 x 4.55 mm ) sized CMOS sensor .
Nikon S9900's sensor provides 6MP more than Sony S950'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 S950 and S9900 sensor size comparison.
Sony S950 and Nikon S9900 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Sony S950 has 32% larger pixel area compared to Nikon S9900, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.