Samsung NX500 has a 28.0MP APS-C (23.5 x 15.7 mm ) sized BSI-CMOS sensor and features DRIMe 5 processor. On the other hand, Samsung NX300 has a 20.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor and features DRIMe IV processor. Samsung NX500's sensor provides 8MP more than Samsung NX300'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.
Another difference between these two cameras is that Samsung NX500's sensor lacks anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the NX500 and NX300 sensor size comparison.
Samsung NX500 and Samsung NX300 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Samsung NX300 has 40% larger pixel area (18.48µm2 vs 13.18µm2) compared to Samsung NX500, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.