Leica SL2-S has a 24.0MP Full frame (36 x 24 mm ) sized BSI-CMOS sensor and features Maestro III processor. On the other hand, Leica SL2 has a 47.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Maestro III processor.
Leica SL2's sensor provides 23MP more than Leica SL2-S'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 similarity between these two cameras is that both Leica SL2 and Leica SL2-S sensors lack anti-alias (Low-Pass) filters. 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 SL2-S and SL2 sensor size comparison.
Leica SL2-S and Leica SL2 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 SL2-S has 94% larger pixel area compared to Leica SL2, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.