Canon SL2 has a
24.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features DIGIC 7 processor. On the other hand, Canon M6 MII has a
33.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features DIGIC 8 processor.
Canon M6 MII's sensor provides 9MP more than Canon SL2'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 SL2 and M6 II sensor size comparison.
Canon SL2 and Canon M6 MII 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 Canon SL2 has 34% larger pixel area compared to Canon M6 MII, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.