To expand on the above comments with my $0.02 worth, I used to own a Lexicon MC-12V5EQ. It's surround processing capabilities are impressive. Mine had both Logic 7 and DPLIIx, which are the premiere processing capabilities for 7.1.
From my perspective, room size is not an issue in deciding on 7.1 verus 5.1. My own room is relatively small, at 13' x 17' x 9.5', and I found a significant improvement in surround envelopment when I expanded from 5.1 to 7.1. However, room layout can be an issue. Your room is relatively large at 16 x 25, but without room behind your listening position, then 7.1 is not a good choice.
However, do you have any flexibility in relocating your sofa so there is room behind the listening position? If so, then I would definitely recommend going to 7.1 as that is a particular sweet spot for the Lexicon.
My former speaker system had tripoles for sides, very similar to using dipoles for sides, and monopoles for rears and that combination does provide a good surround environment. When I recently upgraded my speaker system, the new line did not offer dipoles / bipoles / tripoles, so I was limited to monopoles for both sides and rears. So far, I have not found this be a problem, although if I had the option I would have chosen dipoles / bipoles / tripoles for the sides.
FWIW, I listen to everything in 7.1, independent of source.