Leica M-E Typ 220 has a 18.0MP Full frame (36 x 24 mm ) sized CCD sensor . On the other hand, Leica M Typ 240 has a 24.0MP Full frame (36 x 24 mm ) sized CMOS sensor .
Leica M Typ 240's sensor provides 6MP more than Leica M-E Typ 220'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.
Below you can see the M-E Typ 220 and M Typ 240 sensor size comparison.
Leica M-E Typ 220 and Leica M Typ 240 have 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 M-E Typ 220 has 30% larger pixel area compared to Leica M Typ 240, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It 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. Leica M-E Typ 220 and Leica M Typ 240 sensors have been tested by DxO and the results show that M Typ 240 has a better overall score of 84, 15 points higher compared to M-E Typ 220's score of 69.