If your ceiling is low (<8 feet) and/or aesthetics are a concern, the M3 in-ceiling is the way to go.
In-wall M3s into the ceiling, and somehow angled towards MLP, would be better.
If you want the absolute best, full-cabinet speakers, somehow mounted to the ceiling and pointed towards MLP are the way to go.
I'm perfectly happy with my 4 in-ceilings, which are far better than the bounced Atmos effect speakers any day of the week.
Given the way they mount, not sure how you could angle an in-wall (or in-ceiling).
Since my ceiling has room for either in-celing or in-wall and given they are the same price, I had might as well go in-wall in my ceiling.
My joists run front to back, so the speakers would be mounted length wise in relation to the MLP. Should I mount them so the tweeters are closest to the MLP or so the woofer are? Or should they all be mounted in the same orientation?