Hey Chris

Based upon your wish to have the biggest audio impact, IMHO going 7.1 is what you want, all things being optimized to support the side speakers. Of course you may be considering a two rear surrounds configuration, rather than sides. I have no experience. I'm sure other HHs have plenty of such experience.

Again depending on your system to support 7.1, in my experience the sides increase the size of the sweet spot, enhance front to rear or vice versa audio effects like the most recent THX movie credit and increase the spaciousness of the sound stage.

If your processor is a 7.1 configured M product with MRC, the 7.1 configuration process gives you a great start to fine tuning the mix. After that, you'll probably want to tweak the volume levels to optimize the over all sound. The down side is additional cost. That's my best advice. If supported right, you won't be disappointed.

Tim