Pentax KP has a
24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features PRIME IV processor. On the other hand, Pentax ist DL2 has a
6.0MP APS-C (23.5 x 15.7 mm ) sized CCD sensor .
Pentax KP's sensor provides 18MP more than Pentax ist DL2'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 KP and ist DL2 sensor size comparison.
Pentax KP and Pentax ist DL2 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.