I have a DD-10 and really like it. My room is a bit smaller
than yours (10x10) and the DD-10 is more than enough.I think
Bob makes some good points. My one concern would be that
the DD-12 could be too much for the room. My friend had
a DD-10 and then upgraded to the DD-12. The difference was
not subtle. Yes the DD-12 is better but it might also be
harder to integrate into a smaller room. Your best bet would
be to demo them both and see which one works best.
Good luck