Canon S200 has a 10.0MP 1/1.7" (7.44 x 5.58 mm ) sized CCD sensor and features Digic 5 processor. On the other hand, Canon S120 has a 12.0MP 1/1.7" (7.44 x 5.58 mm ) sized BSI-CMOS sensor and features Digic 6 processor.
Below you can see the S200 and S120 sensor size comparison.
Canon S200 and Canon S120 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 S200 has 20% larger pixel area compared to Canon S120, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.