Leica M Typ 240 has a 24.0MP Full frame
(36 x 24 mm ) sized CMOS sensor . On the other hand, Leica M9 has a 18.0MP Full frame
(36 x 24 mm ) sized CCD sensor .
Leica M Typ 240's sensor provides 6MP more than Leica M9'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.
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 Typ 240 and M9 sensor size comparison.
Leica M Typ 240 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. On the other hand, since Leica M9 has 30% larger pixel area (47.73µm2 vs 36.50µm2) compared to Leica M Typ 240, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.
DxOMark Sensor Scores
Both Leica M Typ 240 and Leica M9 sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, M Typ 240 has scored 84, 15 points higher than M9.
Leica M Typ 240