Samsung NX300M has a
20.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor and features DRIMe IV processor. On the other hand, Nikon D90 has a
12.0MP APS-C (23.6 x 15.8 mm ) sized CMOS sensor .
Samsung NX300M's sensor provides 8MP more than Nikon D90'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 NX300M and D90 sensor size comparison.
Samsung NX300M and Nikon D90 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.