The quadpolar design of the QSs will give you a better surround feeling than the direct firing M2s as it will spread the sound across the room instead of directing it to the listener.
As for the center, if you can fit it, from what I read a M2 would be a better choice, better horizontal dispersion and better tonal match to your mains.
+1.
Someone recently compared a vertical M2 to a VP150 and the M2 got the better of the duel. I use an M2 as centre channel (based on advice from senior members of this board) and have no complaints. It was a great decision.
Go with the QS8.
