Nikon L31 has a 16.0MP 1/2.3' (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Nikon L21 has a 8.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Expeed C2 processor.
Nikon L31's sensor provides 8MP 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 L31 and L21 sensor size comparison.
Nikon L31 and Nikon L21 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.