Nikon S9100 has a 12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Expeed C2 processor. On the other hand, Canon SX170 IS has a 16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4 processor.
Below you can see the S9100 and SX170 IS sensor size comparison.
Nikon S9100 and Canon SX170 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 Nikon S9100 has 32% larger pixel area compared to Canon SX170 IS, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.