I think you are going to have to make the purchases and decide for yourself, they are both good subs and have a slightly different sound. The Axiom will blend with your system like no other sub and it has a nice punch to it, not heavy and should be a perfect match aesthetically to the rest of the system.
I haven't heard the Hsu you are looking at, but the STF3 I have heard had more impact/punch than the 350, IMO(just going from memory, no side by side comparisons), which to me is better for HT. The STF3 was good with music but I had a hard time getting it to blend smoothly with my Axioms, but that could have just been positioning of the sub.
Personally, if it was I who had to chose between these two subs, I would go Axiom.
