It's not so much as which is better as some find the rear surrounds to be better if they are direct radiating, M2's or M22's, while others like the diffuse sound of the QS8s. It is a question of which will you like better and only you can find this out.
IMO, if the listener is far away, say about the same distance or more from the mains, then direct radiating would work better. As you are not far from the rear channels I would think more QS8s would work very well. I have a 6.1 system and have a QS8 for the rear channel and it really spreads the sound around filling out the center rear between the side surrounds.
