The STF models are designed for maximum output whereas the VTF models have a user selectable (maximum output, or maximum extension) modes. The VTF-2 performs no better than the STF-3 in terms of low frequency response but the STF-3 exceeds the VTF-3 in terms of output level capability. The VTF-3 is the best of both worlds, can play loud AND reaches below 20Hz flat.
So the VTF models are better in that you can custom tailor the sound for your needs. In the grand scheme of things, the VTF-3 is what you want. However, it is a BIG sub.
Fans of Hsu says that SVS subs are less "musical".
You don't really have to be sceptical because the SVS actual performance is well documented. However, just note that the SVS subs are quite big....wide, and deep, and that's how they got their good performance. All other things being equal, the bigger the box, the more bass you can get out of it.. I prefer Hsu for the reason they are more tall and narrow for the same volume, takes up less floorspace.
I still think you are better off buying a Mirage, PSB, Energy, or Paradigm sub in Canada. After discounts, you can do just practically as good, and it's really nice having the support of a local dealer. I had to return a set of brand new speakers to my dealer after they developed a buzz after three weeks...it cost me nearly nothing to drive to my dealer and have him give me a new set. I can't imagine shipping a 80LB sub back to the USA. Also, some dealers even have trade up policies. Certainly if you have a problem with the sub, it's is far better to deal with a Canadian distributor than directly with SVS in the USA.
I would't hesitate to buy a Hsu just because they have a Canadian distribution network.....and you don't have to pay extra UPS brokerage fees like you do importing an SVS.