Canon ELPH 350 HS has a 20.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features DIGIC 4+ processor. On the other hand, Olympus TG-6 has a 12.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features TruePic VIII processor.
Canon ELPH 350 HS's sensor provides 8MP more than Olympus TG-6's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the 350 HS and TG-6 sensor size comparison.
Canon ELPH 350 HS and Olympus TG-6 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 Olympus TG-6 has 68% larger pixel area (2.34µm2 vs 1.39µm2) compared to Canon ELPH 350 HS, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.