Nikon L31 has a 16.0MP 1/2.3' (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Canon SD780 IS has a 12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the L31 and SD780 IS sensor size comparison.
Nikon L31 and Canon SD780 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 SD780 IS has 32% larger pixel area (2.34µm2 vs 1.76µm2) compared to Nikon L31, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.