Both Leica M-Monochrom and Leica M9 have Full frame sized 18.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
One other difference between these two cameras that is worth mentioning is that Leica M9's sensor doesn't have an 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 M-Monochrom and M9 sensor size comparison.
Sensor Size and Resolution Comparison image of Leica M-Monochrom and Leica M9 Cameras
Leica M-Monochrom and Leica M9 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.
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 M-Monochrom
or the
Leica M9 ,
including hyperfocal distance.
