Canon SX160 IS has a 16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4 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.
Below you can see the SX160 IS and SX150 IS sensor size comparison.
Canon SX160 IS 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 14% larger pixel area (2.01µm2 vs 1.76µm2) compared to Canon SX160 IS, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.