Canon G12 has a 10.0MP 1/1.7-inch (7.44 x 5.58 mm ) sized CCD sensor and features Digic 4 processor. On the other hand, Nikon L28 has a 20.0MP 1/2.3-inch (6.16 x 4.62 mm ) sized CCD sensor .
Nikon L28's sensor provides 10MP more than Canon G12'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 G12 and L28 sensor size comparison.
As seen above, Canon G12 has a 1.5x Larger sensor area than Nikon L28. 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.