Second rearview Camera mounting point?

I'm putting in a rearview mirror mounted dash cam. It comes with a rear view camera and I'm thinking of mounting it at the top of the back hatch or on the roof. Any thoughts which one is better?
 
I am surprised no one has an opinion on this. With the popularity of dash cams with backup cameras, mounting the camera to compliment the factory camera seems like it would be a common consideration.
 
We have front and rear dash cams. On the rear, we just put it in the upper center of the rear window
 
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