Canon 5DS has a 51.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Dual DIGIC 6 processor. On the other hand, Sony A99 has a 24.0MP Full frame (35.8 x 23.8 mm ) sized CMOS sensor and features Bionz processor.
Canon 5DS's sensor provides 27MP more than Sony A99'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 5DS and A99 sensor size comparison.
Canon 5DS and Sony A99 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. Canon 5DS and Sony A99 sensors have been tested by DxO and the results show that A99 has a better overall score of 89, 2 points higher compared to 5DS's score of 87.