Nikon S6400 has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Expeed C2 processor. On the other hand, Canon M5 has a
24.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 7 processor.
Canon M5's sensor provides 8MP more than Nikon S6400'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 S6400 and M5 sensor size comparison.
As seen above, Canon M5 has a 11.8x Larger sensor area than Nikon S6400. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.