Nikon D700 has a 12.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Expeed processor. On the other hand, Nikon D750 has a 24.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 4 processor.
Nikon D750's sensor provides 12MP more than Nikon D700'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 D700 and D750 sensor size comparison.
Nikon D700 and Nikon D750 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.
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 D700 and Nikon D750 sensors have been tested by DxO and the results show that D750 has a better overall score of 93, 13 points higher compared to D700's score of 80.