Canon 5D MIII has a 22.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Digic 5+ processor. On the other hand, Sony A7S has a 12.0MP Full frame (35.8 x 23.9 mm ) sized CMOS sensor and features Bionz X processor. Canon 5D MIII's sensor provides 10MP more than Sony A7S'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 5D MIII and A7S sensor size comparison.
Canon 5D MIII and Sony A7S 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.