Compare M6 II vs. 5DIV both at 10ft, as this is often the real world situation. You cannot get closer to the subject without one of the following:

a) Falling from cliff, in river, off boardwalk, etc.
b) Scaring "it" off
c) Making yourself "it"'s lunch.
d) View is obscured from further away (trees, crowd, etc.)
... You get the idea.

Considering this 10ft vantage point, what gets me a better image? Possible comparisons - Crop 5DIV to match M6II (which gives best detail at 100%). Downscale M6II to match center of 5DIV (which gives best image for a equally sized print, assuming you're cropping the 5DIV to 1.6).

Your 10ft vs. 16ft test is the opposite of c), and/or reducing the odds of b). It's the "I don't feel comfortable with how close I am, can I move further away if I switch bodies?" / "Can I make my subject more comfortable by moving away?" situation. This is also a good test.