Sigma SD1 Merrill has a
15.0MP APS-C (24 x 16 mm ) sized CMOS (Foveon X3) sensor and features Dual True II processor. On the other hand, Panasonic G9 has a
20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
Panasonic G9's sensor provides 5MP more than Sigma SD1 Merrill'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.
Another similarity between these two cameras is that both Panasonic G9 and Sigma SD1 Merrill sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the SD1 Merrill and G9 sensor size comparison.
As seen above, Sigma SD1 Merrill has a 1.7x Larger sensor area than Panasonic G9. 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.