Nikon 1 J5 has a 21.0MP 1" (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features Expeed 5A processor. On the other hand, Nikon 1 J3 has a 14.0MP 1" (13.2 x 8.8 mm ) sized CMOS sensor .
Nikon 1 J5's sensor provides 7MP more than Nikon 1 J3'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 difference between these two cameras is that Nikon 1 J5's sensor lacks anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the J5 and 1 J3 sensor size comparison.
Nikon 1 J5 and Nikon 1 J3 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 Nikon 1 J3 has 45% larger pixel area (8.20µm2 vs 5.62µm2) compared to Nikon 1 J5, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.
DxOMark Sensor Scores
Both Nikon 1 J5 and Nikon 1 J3 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, J5 has scored 65, 13 points higher than 1 J3.