I use the QS8's as sides and QS4's as backs in my 7.1 setup, they sound almost the same to me, both do an excellent job. I have never done an AB comparison on them, but I would guess by the specs alone the 8's will handle much more volume. If I was only using a 5.1 system I would definitely go with the QS8's.

A computer once beat me at chess, but it was no match for me at kick boxing.