Canon M6 MII has a 33.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features DIGIC 8 processor. On the other hand, Canon M6 has a 24.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 7 processor.
Canon M6 MII's sensor provides 9MP more than Canon M6'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 M6 II and M6 sensor size comparison.
Canon M6 MII and Canon M6 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 Canon M6 has 34% larger pixel area (13.84µm2 vs 10.29µm2) compared to Canon M6 MII, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.