I have been pondering your question and I think if I had to make that decision--I would go with the sw-15. I have paired the Sony in the past with another sub and it did work well for the money, but one better sub with lower extension will serve you better imo.
I already have one Sony SA-WM40, and my room is around 13x13, and don't know if the sw-15 would be too much. It puts out way more power than I need of around 200 rms for my room, compared to the sony 120 rms. Since I already have one, adding another would get me 240 rms, as opposed to 320 rms! I would never use all of its potential, and I can get the sony sub for $80 cheaper. What do you think?
Neal I see your train of thought. Most of us "do not" use our sub-subs to it's full potential. You do want head-room and you don't want to be straining your sub. With settings set to high you would have distortion and run the risk of bottoming out not to mention being way out of calibration. Two Sony's will give you more head-room and the SW-15 would provide the same at lower extension. The 15 would not be to much. You could go with a SVS or HSU and that would OK. It is not just volume but the quality of that volume and how low will it go- Extension. If you do HT mostly and like to run your sub a little hot, -localization- two Sony's will work. Since you have one Sony already its a decision only you can make "if you decide to take the mission"?