Canon SX240 HS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Digic 5 processor. On the other hand, Canon SX740 HS has a
21.0MP 1/2.3 (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features DIGIC 8 processor.
Canon SX740 HS's sensor provides 9MP more than Canon SX240 HS'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 SX240 HS and SX740 HS sensor size comparison.
Canon SX240 HS and Canon SX740 HS 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 SX240 HS has 68% larger pixel area compared to Canon SX740 HS, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.