Only my experience, but here's my thought processes when I made a similar decision. The VTF 15 wasn't announced yet, so the top of the line HSU ported sub available at the time was the VTF3 mk3. I compared it to everything i could find to the point of 'analysis paralysis'. Comparing reviews and graphs and others' subjective comparisons, I was really having a difficult time... I started looking into DIY options (no time), scouring craigslist (used crap others were upgrading from), ebay ( no thanks ), and was ready to just say screw it, just step away, take a deep breath, look at it again in 6 months. Then I found Audiogon. The first thing I noticed, was nothing for sale with a HSU label. Nothing. Plenty from everyone else...
I emailed Hsu, Rythmik, Svs, Ed, and Cht, and the ONLY one that responded (very promptly, at that) was Pete at Hsu. I ended up on multiple email lists from the other companies mentioned, but never did get an answer to my original questions...
I did watch the used market for a while, and whenever i did see a Hsu sub pop up, which wasn't often, it sold VERY quickly, at least compared to listings for other ID manufacturers. Combined with their great customer service, and pretty much complete lack of negative reviews, I placed my order. One VTF3 mk 3, please. Ordered late on a Friday afternoon, at my door Tuesday.
It is an amazing sub. I don't have any listening experience with the others, but I have no regrets whatsoever. Coming from an Onkyo Htib sub to this was beyond amazing. Being able to truly FEEL the music, like it's coming from your bones vibrating, and movies... first movie I put in was Gamer, and I didn't think I'd make it through 5 minutes of that movie without my house crumbling around me.
I can't compare the 'sonic capabilities' of my Hsu to the other brands, but I do see why so few ever pop up on the used market. Mine wont be for sale for a loooong time.
Hope it helps,