Sony A100 has a 10.0MP APS-C (23.6 x 15.8 mm ) sized CCD sensor . On the other hand, Sigma DP3 Merrill has a 15.0MP APS-C (24 x 16 mm ) sized CMOS (Foveon X3) sensor and features Dual TRUE II engine processor.
Sigma DP3 Merrill's sensor provides 5MP more than Sony A100'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 A100 and DP3 Merrill sensor size comparison.
Sony A100 and Sigma DP3 Merrill 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.