Nikon D5 has a 21.0MP Full frame (35.8 x 23.9 mm ) sized CMOS sensor and features Expeed 5 processor. On the other hand, Leica SL has a 24.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Maestro II processor.
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 moire occurring in certain scenes.
Below you can see the D5 and SL sensor size comparison.
Nikon D5 and Leica SL have almost the same sensor size so none of them has any significant advantage over other in this area.