The HSU will probably sound better for music, but may slightly lack in overall output to the A2-300. The A2-300 digs very deep for its price. It would really boil down to how cheap are we talking for the VTF2-Mk3? If they are the same price, I'd prolly go with the HSU VTF2-Mk3.
If you're looking at an A2-300 w/ free shipping for $415 and a VTF2-Mk3 for $509+ shipping....the A2-300 is still the better deal and unless you're a critical (i.e reference bookshelf) listener, the A2-300 seems it would be a better fit.
Frankly, if you're that distraught, I would up and pay the $699+ shipping for the VTF3-Mk3:
http://www.hsuresearch.com/products/vtf-3-mk3.html
OR look at the $600 w/ free shipping A3-300 (eD's front firing 12" sub w/ 300 watt RMS amp and an upgraded driver with 25mm/1 inch one-way excursion):
http://www.edesignaudio.com/product_info.php?cPath=2_41&products_id=102
The VTF3-Mk3 has an XBL-2 woofer design that has much longer and more linear excursion than the VTF2-Mk3. It would be worth the additional funds between the two, even at the VTF2-Mk3's father's day sale price, IMHO. The eD A3-300 sounds very similar to the SVS PB12-NSD, but plays louder and slightly lower while listening to my buddy's test disc. The SVS PB12-NSD would also be an option.
I owned an 'original' VTF-2 w/ the 10" woofer and sold it to my brother a while back in favor of my A2-300 because I moved into a bigger living room. Now he loves using it...it's gonna be 8 years old soon.

The A2-300 just had more excursion and plenty of power to put it to use. There are a couple reviews of the A2-300 out there, but they are all of units with the older amplifier, old cabinet, and older aluminum driver....so there isn't a wealth of reviews on the new A2-300.
Lemme know if you have any specific questions about the A2-300.