Nikon D300S has a
12.0MP APS-C (23.6 x 15.8 mm ) sized CMOS sensor and features Expeed processor. On the other hand, Pentax KP has a
24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features PRIME IV processor.
Pentax KP's sensor provides 12MP more than Nikon D300S'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 D300S and KP sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon D300S and Pentax KP Cameras
Nikon D300S and Pentax KP have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Nikon D300S
or the
Pentax KP ,
including hyperfocal distance.