Sony HX400V has a 20.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Bionz X processor. On the other hand, Panasonic FZ300 has a 12.0MP 1/2.3 (6.17 x 4.55 mm ) sized CMOS sensor and features Venus Engine processor. Sony HX400V's sensor provides 8MP 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 HX400V and FZ300 sensor size comparison.
Sony HX400V and Panasonic FZ300 sensors have exactly the same size so none of them has any advantage over other in this area.