Canon 5DS R has a 51.0MP Full frame
(36 x 24 mm ) sized CMOS sensor and features Dual DIGIC 6 processor. On the other hand, Canon 5D MII has a 21.0MP Full frame
(36 x 24 mm ) sized CMOS sensor and features Digic 4 processor.
Canon 5DS R's sensor provides 30MP more than Canon 5D MII'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.
Another difference between these two cameras is that Canon 5DS R's sensor lacks anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the 5DS R and 5D MII sensor size comparison.
Canon 5DS R and Canon 5D MII have the same 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 5D MII has 139% larger pixel area (41.08µm2 vs 17.17µm2) compared to Canon 5DS R, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.
DxOMark Sensor Scores
Both Canon 5DS R and Canon 5D MII sensors have been tested by DxoMark. DxoMark 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. Of the two cameras that we are comparing, 5DS R has scored 86, 7 points higher than 5D MII.
Canon 5DS R
Canon 5D MII