The VTF15h km2 is a bit over $1K when you factor in shipping. The VTF-3 is like $900.
The inclusion of the Dayton doesn't make this a very simple comparison, because we don't know what amp you will be using, nor do we know what type of performance you are interested in. The Dayton Ultimax 18" can give you more overall performance- but only if you are using one hell of an amplifier. Even with a beast of an amp, the Ultimax doesn't have an advantage from, say 16 Hz to 31 Hz against the ported Hsu 15" subs. However, above and below that range, it will have an advantage, but you need a beefy amp to get more output than the Hsu subs at 40 Hz and above. I think you will want something like a Behringer 3000DSP amp with the Ultimax 18", at the least, but even that won't give it a big mid-bass advantage over the Hsu subs.