i am trying to decide between the qs4, and qs8. Based on the room dimensions below, which would work best? I realize there is a significant difference in price, and I am trying to determine if it is justified based on room size and performance:

room is approx 19.5ft long by approx 15.5 wide. One 15.5ft wall has my Mits 55" widescreen with center speaker on top.
the back 15.5ft wall has two surrounds on wall, with the main couch approx 3 ft in front of the surrounds. I would like to either replace the polk sattelites in back with Axiom qs4 or qs8 and move the polks to the side(hang from ceiling), or leave the polks in back and hang the axioms on the side, or, just replace the polks in back with axioms, and leave the 5.1 setup. Any help would be appreciated!