Nikon P500 has a 12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Expeed C2 processor. On the other hand, Canon A3500 IS 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 P500 and A3500 IS sensor size comparison.
Nikon P500 and Canon A3500 IS 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 Nikon P500 has 32% larger pixel area compared to Canon A3500 IS, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.