Canon G15 has a 12.0MP 1/1.7" (7.44 x 5.58 mm ) sized CMOS sensor and features Digic 5 processor. On the other hand, Canon G12 has a 10.0MP 1/1.7" (7.44 x 5.58 mm ) sized CCD sensor and features Digic 4 processor.
Below you can see the G15 and G12 sensor size comparison.
Canon G15 and Canon G12 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Canon G12 has 20% larger pixel area (4.16µm2 vs 3.46µm2) compared to Canon G15, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It 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. Canon G15 and Canon G12 sensors have been tested by DxO and the results show that G12 has a better overall score of 47, 1 points higher compared to G15's score of 46.