No. the sixth channel in a 6.1 set-up is not a discrete channel--it is just the two rear channels added together, thus it really adds nothing, particularly given the dimensions of your room. 5.1 is the standard and everything is mixed for 5.1, so I would stick with 5.1 and not worry about it. In fact, you would hear more and better sound buying five higher quality speakers than 6 speakers of lesser quality.
Actually the 6th channel is discrete but there are not to many DVD's out their that have the discrete ES channel mixed in. Rush Hour 2 is an example of a true 6.1 movie. I run a 7.1 channel set-up and love it, the back 2 channels are matrixed in a 7.1.
maybe it's because there isn't any true 7.1 DVD disk now. Let's wait when both the 6.1 and 7.1 format disks are all formally available and see if there is difference.
I'd recommend buying a 5.1 speaker setup and a 6.1 amp and you can always buy 1 more speaker later. Remember, the rear speakers are more fill and effect than anything. I have a room about your size and 5.1 does awesome.
I have a 600watt,7piece home theater audio system w/3-dvd/cd changer.If I take away 5 of theses speakers,Can I hook up a 400watt speakers that carry 12" woofers/4" Midrange/and a high SPL Professional tweeter horns(3-way P.A./D.J./Nightclub speakers).Please let me know before I purchase....Thank You !!!!!!