Canon 50D has a 15.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 4 processor. On the other hand, Canon 40D has a 10.0MP APS-C (22.2 x 14.8 mm ) sized CMOS sensor .
Canon 50D's sensor provides 5MP more than Canon 40D'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 50D and 40D sensor size comparison.
Canon 50D and Canon 40D have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.
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. Canon 50D and Canon 40D sensors have been tested by DxO and the results show that 40D has a better overall score of 64, 1 points higher compared to 50D's score of 63.