Sony A9 has a 24.0MP Full frame (35.6 x 23.8 mm ) sized BSI-CMOS sensor and features BIONZ X processor. On the other hand, Nikon D2H has a 4.0MP APS-C (23.7 x 15.5 mm ) sized JFET sensor .
Sony A9's sensor provides 20MP more than Nikon D2H'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 A9 and D2H sensor size comparison.
As seen above, Sony A9 has a 2.3x Larger sensor area than Nikon D2H. 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 Sony A9 and Nikon D2H 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, A9 has scored 92, 52 points higher than D2H.