The STF-2 will not be better than the VTF-2. It is supposed to sound exactly the same as the VTF-2, but it won't be as adjustable. It will just be cheaper.

VTF stands for Variable Tuning Frequency. That model of subwoofer allows you to plug (max extension mode) or unplug (max output mode) one of its two ports to customize it's output to your liking.

STF stands for Set Tuning Frequency and this series has only one port. That's really the only difference between the two.