Canon SD780 IS has a 12.0MP 1/2.3" (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Sony TX66 has a 18.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features BIONZ processor.
Sony TX66's sensor provides 6MP 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 SD780 IS and TX66 sensor size comparison.
Canon SD780 IS and Sony TX66 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 49% larger pixel area compared to Sony TX66, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.