Pentax X90 has a
12.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Prime processor. On the other hand, Pentax K100D S has a
6.0MP APS-C (23.5 x 15.7 mm ) sized CCD sensor .
Pentax X90's sensor provides 6MP more than Pentax K100D S'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 X90 and K100D Super sensor size comparison.
As seen above, Pentax K100D S has a 13.3x Larger sensor area than Pentax X90. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.