Sony A65 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz processor. On the other hand, Sigma Quattro has a 29.0MP APS-C (23.5 x 15.6 mm ) sized CMOS (Foveon X3) sensor and features Dual TRUE III processor. Sigma Quattro's sensor provides 5MP more than Sony A65'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 A65 and Quattro sensor size comparison.
Sony A65 and Sigma Quattro sensors have exactly the same size so none of them has any advantage over other in this area.