Both Canon G12 and Nikon P7100 have 1/1.7" sized 10.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Below you can see the G12 and P7100 sensor size comparison.
Canon G12 and Nikon P7100 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.
DxOMark Sensor Scores
Both Canon G12 and Nikon P7100 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, G12 has scored 47, 6 points higher than P7100.