Canon 1Ds MIII has a 21.0MP Full frame (36 x 24 mm ) sized CMOS sensor . On the other hand, Canon 5DS has a 51.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Dual DIGIC 6 processor. Canon 5DS's sensor provides 30MP more than Canon 1Ds MIII'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 1Ds MIII and 5DS sensor size comparison.
Canon 1Ds MIII and Canon 5DS have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Canon 1Ds MIII has 139% larger pixel area compared to Canon 5DS, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.