Nikon L21 has a
8.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Expeed C2 processor. On the other hand, Canon SX30 IS has a
14.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4 processor.
Canon SX30 IS's sensor provides 6MP more than Nikon L21'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 L21 and SX30 IS sensor size comparison.
Nikon L21 and Canon SX30 IS have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.