Leica SL3-S has a
24.0MP Full frame (36 x 24 mm ) sized BSI-CMOS sensor and features Maestro IV processor. On the other hand, Nikon D6 has a
21.0MP Full frame (35.9 x 23.9 mm ) sized CMOS sensor and features Expeed 6 processor.
Another difference between these two cameras is that Leica SL3-S'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 SL3-S and D6 sensor size comparison.
Sensor Size and Resolution Comparison image of Leica SL3-S and Nikon D6 Cameras
Leica SL3-S and Nikon D6 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Leica SL3-S
or the
Nikon D6 ,
including hyperfocal distance.