Rustolem, the QS8s will produce a more disperse sound which, in most cases, should be a bit more enveloping. Direct radiating speakers, like the M3s can work as surrounds too, but will be a bit more directional. If you plan on doing mostly multichannel music the M3s may work better for you. If it's mostly HT the QS8s may work better.

Preference and room acoustics may also sway the decision. The QS8s are pretty forgiving in terms of placement.

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"Nothin' up my sleeve. . ." --Bullwinkle J. Moose