It's not just 8" vs 10" vs 12" vs 15" (that's car audio crazy talk lol) You can put an 8" woofer in a horn loaded box and it will rearrange your furniture.
That said, the goal is to have a good audible response from 20-80hz, which is where home theater and music bass exists. The STF 1, and 2 both fall off shy of 20 hz. they also have limited power to play particularly loud or fill up a medium to large size room.
For the HSU line, if you are doing HT, and music, I suggest saving up for the VTF 2. It covers all the bases.