FujiFilm S2800HD has a 14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Sigma SD15 has a 5.0MP APS-C (20.7 x 13.8 mm ) sized CMOS (Foveon X3) sensor and features True II processor.
FujiFilm S2800HD's sensor provides 9MP more than Sigma SD15'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 SD15 sensor size comparison.
As seen above, Sigma SD15 has a 10.2x Larger sensor area than FujiFilm S2800HD. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.