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. On the other hand, Canon SX400 IS has a
16.0MP 1/2.3 (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4+ processor.
Below you can see the SX150 IS and SX400 IS sensor size comparison.
Canon SX150 IS and Canon SX400 IS 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 SX150 IS has 14% larger pixel area compared to Canon SX400 IS, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.