Canon 70D has a 20.0MP APS-C (22.5 x 15 mm ) sized CMOS sensor and features Digic 5+ processor. On the other hand, Canon 50D has a 15.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 4 processor.
Canon 70D's sensor provides 5MP more than Canon 50D'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 70D and 50D sensor size comparison.
Canon 70D and Canon 50D 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
Both Canon 70D and Canon 50D 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, 70D has scored 68, 5 points higher than 50D.