Ricoh WG-6 has a 20.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor . On the other hand, Nikon P90 has a 12.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor .
Ricoh WG-6's sensor provides 8MP more than Nikon P90'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 WG-6 and P90 sensor size comparison.
Ricoh WG-6 and Nikon P90 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.