Canon G9 X has a 20.0MP 1" (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features DIGIC 6 processor. On the other hand, Canon G15 has a 12.0MP 1/1.7" (7.44 x 5.58 mm ) sized CMOS sensor and features Digic 5 processor.
Canon G9 X's sensor provides 8MP more than Canon G15'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 G9 X and G15 sensor size comparison.
As seen above, Canon G9 X has a 2.8x Larger sensor area than Canon G15. 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.
DxOMark Sensor Scores
Both Canon G9 X and Canon G15 sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, G9 X has scored 63, 17 points higher than G15.