I own a STF-2 and I really enjoy it. I was on a limited budget so that's what I could afford at the time via a used model.
One thing I later realized though is that the STF-2 only gets down to 25hz (same with the VTF-1 in ME mode, I believe), so that is a factor to consider, especially for movie watching.
The VTF-2 gets much lower so I would imagine there would be improved movie watching experience with the subsonic frequencies. If I had the money at the time, I would have bought the VTF-2.
Anyway, I put the STF-2 in my bedroom system which is primarily music (although I do watch movies there, too), and just bought a bigger sub for the HT room, an Outlaw LFM-1 EX, which is comparable to the VTF-3.
It was advised to me from posters and by Hsu to buy the biggest sub that you can afford at the time, and I fully understand that now. STF-2 is a very nice sub, better than my previous. I don't think you'd go wrong with the STF-2/VTF-1 as movie watching is still fun, but you may be missing out a bit on the experience with a sub that can go a little lower, as you'll start feeling the low frequencies more, and from what I understand, really enhances the movie-going experience. It's the main reason why I upgraded my subwoofer. I don't play that loud either. Hopefully my new subwoofer will arrive soon, so I can know for sure, but based on what others have posted, from going from a 10 inch sub to one that can play lower frequencies with more ouput, the difference should be significant.
One thing you can consider is buying an eD A2-300, which provide a lot of low-end bang for the buck. I think you lose some articulation for music as compared to the Hsu/Outlaw, but you get comparable to low-end slam to the VTF-2 I think. I don't know how long they take to fulfill orders though as sometimes the wait on the eD could be long, so that is factor to consider. Hsu, on the other hand, is known to ship very fast.
If you can afford it, I suggest going with the VTF-2, especially with your room size and if you plan to get another sub.