FujiFilm S2800HD has a 14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Nikon L21 has a 8.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Expeed C2 processor.
FujiFilm S2800HD's sensor provides 6MP more than Nikon L21'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 S2800HD and L21 sensor size comparison.
FujiFilm S2800HD and Nikon L21 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.