Just my opinion, but it depends on what you'll be using this room for. If it's for HT primarily, I'd go with 4 QS8s. They are FABULOUS surround speakers for movies, and actually do an excellent job when just listening to music in any 5.1, 6.1 or 7.1 format.
But, if you're in a situation where you'll be listening to a LOT of Hi Rez disks (SACD & DVD-A) it's not a bad idea to have direct radiating speakers (like the M2) in the rear surround position while still using QS8s as side surrounds for movies. When listening to 5.1 Hi REZ you can select the direct radiating speakers for your surrounds, and select the QS8s as your surrounds for 5.1 movies (DD & DTS).
