Canon ELPH 300 HS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features DIGIC 4 with iSAPS technology processor. On the other hand, Canon ELPH 135 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 ELPH 300 HS and ELPH 135 sensor size comparison.
Canon ELPH 300 HS and Canon ELPH 135 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 ELPH 300 HS has 32% larger pixel area compared to Canon ELPH 135, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.