Nikon D3S has a
12.0MP Full frame (36 x 23.9 mm ) sized CMOS sensor and features Expeed processor. On the other hand, Canon 20Da has a
8.0MP APS-C (22.5 x 15 mm ) sized CMOS sensor .
Below you can see the D3S and 20Da sensor size comparison.
As seen above, Nikon D3S has a 2.5x Larger sensor area than Canon 20Da. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.