Nikon B700 has a 20.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor . 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.
Nikon B700'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 B700 and FZ300 sensor size comparison.
Nikon B700 and Panasonic FZ300 have the same 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 Panasonic FZ300 has 68% larger pixel area (2.34µm2 vs 1.39µm2) compared to Nikon B700, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.