Canon Elph 115 IS has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features DIGIC 5 processor. On the other hand, Canon A3100 IS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the Elph 115 IS and A3100 IS sensor size comparison.
Canon Elph 115 IS and Canon A3100 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 A3100 IS has 32% larger pixel area (2.34µm2 vs 1.76µm2) compared to Canon Elph 115 IS, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.