Panasonic ZS50 has a 12.0MP 1/2.3 (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Panasonic ZS70 has a 20.0MP 1/2.3 (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Venus Engine processor. Panasonic ZS70's sensor provides 8MP more than Panasonic ZS50'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 ZS50 and ZS70 sensor size comparison.
Panasonic ZS50 and Panasonic ZS70 sensors have exactly the same size so none of them has any advantage over other in this area.