I'm thrilled with my STF-2; I'd heartily recommend it, but with this caveat: It's the only sub I've ever heard and owned. Crazy, I know.
I chose it largely on the basis of design (downward firing--which seems to offer more placement flexibility), reputation, and the insights of people who own and enjoy them. It also strikes me as the best value for ~$400.
I notice you're comparing it to the $500 EP175. If I were spending that kind of money, I'd probably consider the Hsu STF-3 or the SVS PB1. (Again, on the basis of so many credible recommendations.) Although, given the size of your room, I'd think the STF-2 or VTF-2 would be A-OK.
All this, of course, presupposes that you're in the US.
HTH,
Roger
