6th/7th channel brings rear surround, vs the approx 90-110 deg. side surround of 5.1 systems.
IMO for many people 6/7 channel rear surround generally isn't worth the cost, esp if you're talking 1 or 2 more QS8s. For music there's essentially no discrete 6.1 or 7.1 material. For HT, there's some, but not much in the context of the total movie catalog. I have HDTV, and it's fairly rare to get 5.1 audio even for HD broadcasts; most is Dolby stereo.
Teddy's HK AVR 630 has Logic 7 to reconstruct 5.1 from stereo sources. I don't know if it has Dolby PLIIx or equivalent to reconstruct 7.1 from 5.1 sources. But if my experience using regular PLII to reconstruct 5.1 from stereo sources is any guide, while a nice feature it's not remotely good as discrete multichannel material. If you don't have PLIIx or equivalent, your rear speaker(s) won't even activate without discrete 6/7 channel material, of which there isn't much.
In general my advice is wire for 7.1 (assuming you're in that installation phase), but just get speakers for 5.1. Use the extra money you'd have spent on the one or two rear QS8s for a better sub, receiver, acoustic room treatment, etc. Make sure your receiver at least has Dolby PLII or HK Logic 7 to reconstruct 5.1 from stereo sources.
If music is a priority, get a combination SACD/DVD or SACD/DVD-A player and some discrete multichannel discs. That would at least use your 5.1 setup.
IOW it's better to have a good 5.1 setup with PLII or L7 for stereo->5.1 upconversion and some discrete multichannel music, than to have a bare bones 6 or 7 channel setup without this.
If you later decide to go 7.1, you're already wired for it and just install the two extra speakers. If you get access to two cheaper speakers (borrowed from a friend, yard sale, etc) you could try out 6/7 channels. If you really like it, then get two more QS8s.
OTOH if you watch recent DVD movies 90% of the time (more likely to have 6/7 channel soundtracks), with little music, TV or older movies, I could see going with a 6/7 channel setup.
