Canon SX20 IS has a 12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4 processor. On the other hand, Sigma DP1s has a 5.0MP APS-C (20.7 x 13.8 mm ) sized CMOS (Foveon X3) sensor .
Canon SX20 IS's sensor provides 7MP more than Sigma DP1s'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 SX20 IS and DP1s sensor size comparison.
As seen above, Sigma DP1s has a 10.2x Larger sensor area than Canon SX20 IS. 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.