Looking to upgrade my EP-175 to something more powerful. I would sell the Axiom to offset the cost.

The Hsu STF-2 is $550, but is the same size driver as my EP-175 and only 25 watts more powerful.

The STF-3 is almost twice as powerful in terms of watts, and is a 12" sub.

Would the -3 be overkill, while the -2 not much of an upgrade?

For a noticable upgrade go with the 3. going to a 12" sub is going to give you more output and deeper extension.

Another option.
For the cost on an STF-2 or 3 and an upgrade credit on your current Axiom sub, you could probably get an Axiom EP-500.

how big is your room?