My experience with 7.1 (which I use) is that is "spreads" the sound out between the four speakers. Or I should say the left surround sound signal is spread out between the left surround and the rear left surround. What this means is that you won't get the full channels 4 and 5 signal PLUS channels 6 and 7, but you'll get a more split sound between the 4 surrounds. Perhaps, overall the same volume. But that only applies to matrixed 5.1 movies. With the quality of the QS8s it's not necessary to use the 7.1 setup unless for a few reasons. You want to have it for 6.1 encoded movies (the rear plays a mono signal), you want to use if for DD-EX movies in which there is special encoding for channels 6 and 7 (LOTRs and Star Wars EP 1 and 2 are good examples), or you have a large room and you want to spread the sound out. In my case I don't have a "huge room", but big enough that now when something comes from "behind" me it REALLY sounds like's it's behind and not side and behind. A great moment in movies is when Aragorn enters King Theoden's chamber in The Two Towers after he's been riding for days...they close the door behind him as he enters the room and it really is directly behind you.
So generally I'm an advocate of 7.1. Though in some cases, at least how my H/K receiver works, I usually like the sound of DTS 5.1 vs DTS-Matrix 7.1. There's subtle differences, but I think that my receiver is making changes to all the channels ever so slightly, and I don't care for that all that much.
So...there's my opinion on it all, for what it's worth