Let's keep it real!
$895 (Hsu 15) is a 28% increase over $699. That is not close to the same price!
Whether the Hsu is worth the extra $196 is a reasonable discussion that has no clear answer as the value of $196 (available for audio budget) is very subjective.
It's not quite as big a jump from the 12" mono to the 15" HSU as you list. Monoprice charges sales tax whereas HSU does not outside it's home state. If you are lucky enough to live in a state with no sales tax it doesn't matter, but if you pay 8.5% as I do it makes a difference.
With that calculation its about $140 extra for the HSU VTF-3 MK5 HP Sub and if you buy it during one of their many HSU sales that occur on just about every holiday it's a $120 difference.
The 15" HSU is a big step up from the 12" mono to my ear. If you have never heard the HSU VTF-3 MK5 HP Sub then you may be totally happy with a 12". If you have a HSU VTF-3 MK5 HP Sub you are probably spoiled like me and wouldn't even think about a 12" mono.