Yeah, a lot of it depends on where your crossover is set. I could see the QS8 and QS4 sounding virtually identical. The QS8 can obviously handle a lot more power, but that is relative as well.
As for the M3 vs QS# question, I loved my QS8s for the many years that I had them, I just knew, from the moment I heard my first Atmos demo that I wanted to have it. Now I am just about a week away from full implementation of it, and it does a lot better with monopole (regular) speakers than a traditionally diffused bi/di/quad-pole speaker (with the QS series being quadpole).
Also, a lot of people, for regular 7.1 setups, prefer monopole speakers (like the on-wall M series) for rear surrounds and QS ones for the sides. You won't go wrong either way in that aspect for traditional 7.1 sound.
