Off hand, I'd think you'd want the QS8s for the surround channel (side) and the M2 for the rear channels.

More films/games utilize 5 channels, with fewer mixed for 7. (even tho your receiver can usually throw your backs a bone on 5 channel sound) Given that, I think the Qs8s are better surround workhorses, especially if most of your material is 5 channel.