We have pretty much given up on telling anyone to save a few bucks and get the one which will be fine for them. Everyone who does that ends up upgrading BEFORE THE FIRST SPEAKERS EVEN COME IN so they don't even get a chance to listen to the VP100, QS4, M50, whatever.
Now we just tell people to buy the one they really want. Seems to work much better for everyone...
EDIT -- Seriously, the problem is that as far as we know nobody on the board has had a chance to compare QS4 and QS8 directly. Axiom says (and they have never steered us wrong yet) that the two speakers sound quite similar so QS4s are the way to go with a moderate sized room and QS8s are the way to go as your room gets larger. Roughly the same deal as VP100 vs. VP150.
There are a few anecdotal hints that the QS4 just might match the M3/M40/M50 family tonally a bit better while the QS8 is closer to M2/M22/M60/M80, but (a) nobody at Axiom has every said anything like that (it's just the way they pair up the speakers in their HT combos), (b) there are only a couple of data points, not enough to have any confidence in the conclusion, and (c) I repeat nobody here has heard the two side by side so we're all just guessing.
The most common pairing does seem to match the Epic pairings, ie QS8s with M22/M60/M80 and QS4s with M3/M40/M50. I run QS8s with M2s which does not follow the pattern but I figure Axiom just felt that nobody would be dumb enough to use $500 surrounds with $300 mains
