Panasonic ZS15 has a 12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Panasonic ZS70 has a 20.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Venus Engine processor.
Panasonic ZS70's sensor provides 8MP more than Panasonic ZS15'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 ZS15 and ZS70 sensor size comparison.
Panasonic ZS15 and Panasonic ZS70 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 Panasonic ZS15 has 68% larger pixel area compared to Panasonic ZS70, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.