Canon SX740 HS has a 21.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features DIGIC 8 processor. On the other hand, Canon SX150 IS has a 14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4 processor.
Canon SX740 HS's sensor provides 7MP more than Canon SX150 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 SX740 HS and SX150 IS sensor size comparison.
Canon SX740 HS and Canon SX150 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 SX150 IS has 44% larger pixel area (2.01µm2 vs 1.39µm2) compared to Canon SX740 HS, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.