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 S640 has a 12.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Expeed processor.
Ricoh WG-6's sensor provides 8MP more than Nikon S640'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 S640 sensor size comparison.
Sensor Size and Resolution Comparison image of Ricoh WG-6 and Nikon S640 Cameras
Ricoh WG-6 and Nikon S640 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.