Panasonic FZ300 has a 12.0MP 1/2.3" (6.17 x 4.55 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Nikon P520 has a 18.0MP 1/2.3-inch (6.16 x 4.62 mm ) sized BSI-CMOS sensor .
Nikon P520's sensor provides 6MP more than Panasonic FZ300'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 FZ300 and P520 sensor size comparison.
Panasonic FZ300 and Nikon P520 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.