Ricoh G900 has a 20.0MP 1/2.3 (6.17 x 4.55 mm ) sized BSI-CMOS sensor . On the other hand, Pentax X90 has a 12.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Prime processor.
Ricoh G900's sensor provides 8MP more than Pentax X90'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 G900 and X90 sensor size comparison.
Ricoh G900 and Pentax X90 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.