Canon SX210 IS has a
14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4 processor. On the other hand, Canon SX620 HS has a
20.0MP 1/2.3 (6.17 x 4.55 mm) ) sized BSI-CMOS sensor and features DIGIC 4+ processor.
Canon SX620 HS's sensor provides 6MP more than Canon SX210 IS'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 SX210 IS and SX620 HS sensor size comparison.
Canon SX210 IS and Canon SX620 HS 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 SX210 IS has 44% larger pixel area compared to Canon SX620 HS, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.