Canon SD940 IS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4 processor. On the other hand, Canon ELPH 180 has a
20.0MP 1/2.3 (6.17 x 4.55 mm ) sized CCD sensor and features DIGIC 4+ processor.
Canon ELPH 180's sensor provides 8MP more than Canon SD940 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 SD940 IS and ELPH 180 sensor size comparison.
Canon SD940 IS and Canon ELPH 180 have the same 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 SD940 IS has 65% larger pixel area compared to Canon ELPH 180, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.