Leica SL has a
24.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Maestro II processor. On the other hand, Leica M-Monochrom has a
18.0MP Full frame (36 x 24 mm ) sized CCD sensor .
Leica SL's sensor provides 6MP more than Leica M-Monochrom'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 Leica SL'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 SL and M-Monochrom sensor size comparison.
Leica SL and Leica M-Monochrom 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 Leica M-Monochrom has 32% larger pixel area (47.73µm2 vs 36.00µm2) compared to Leica SL, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.