<font color='#000000'>Andreas,
My room is a little bit larger, but it is not a closed area, it opens up into a larger living area. Typical Florida open floor plan. Other subwoofers that I have used, including the Velodyne, caused unwanted vibrations and resonances in my room that were quite annoying. The Paradigm Seismic has such a low resonant frequency that all of these issues have literally disappeared.
Interestingly enough, with very little set-up time, the Paradigm blends seamlessly with the L-C-R speakers. based on my experience with this product I would go for the 12" model. It is not huge, and when something sounds this good I would want as much quantity as possible.
Most subwoofers do not sound good unless crossed over at a very low cut-off frequency. This one, however, does and as a matter of fact I find myself using more of this sub than I have from previous models I have owned.
See if your dealer will let you take home one of the two models and try it out. Even though your room is on the smaller side I feel the 12" model may work just fine. Since it has passive radiators in lieu of ports it seems to be less sensitive of room size.
I hope this rambling helps. Let me know if you have any more questions.
Buddy</font>