Nikon D750 has a 24.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 4 processor. On the other hand, Sony A7R has a 36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Bionz X processor.
Sony A7R's sensor provides 12MP more than Nikon D750'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.
One other difference between these two cameras that is worth mentioning is that Sony A7R's sensor doesn't have an 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 D750 and A7R sensor size comparison.
Nikon D750 and Sony A7R 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 D750 has 49% larger pixel area compared to Sony A7R, it has potential to collect more light on pixel level hence 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. Nikon D750 and Sony A7R sensors have been tested by DxO and the results show that A7R has a better overall score of 95, 2 points higher compared to D750's score of 93.