With a 10 foot distance, I'd go with the M2s. I have a 7.5' distance, and my old Sony surrounds fill in very well with the diffuse QS8s on the sides.
You'll want to place the M2s toward the edges of the listening area on the back wall. For example, I have rears just about lined up with (slightly wider) the arms of the couch.
And as I was saying, with the use of rear surrounds, and Prologic IIx (or DTS NEO:6) you can place the QS8s directly to the sides of the listening area, so the flat between the tweeters is lined up with with the listener's ears.
