Canon SX410 IS has a 20.0MP 1/2.3" (6.17 x 4.55 mm ) sized CCD sensor and features DIGIC 4+ processor. On the other hand, Canon SD780 IS has a 12.0MP 1/2.3" (6.17 x 4.55 mm ) sized CCD sensor . Canon SX410 IS's sensor provides 8MP more than Canon SD780 IS'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 SX410 IS and SD780 IS sensor size comparison.
Canon SX410 IS and Canon SD780 IS 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 Canon SD780 IS has 65% larger pixel area (2.34µm2 vs 1.41µm2) compared to Canon SX410 IS, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.