The piranha PSI is the smartest thing that any low/mid semi maker ever did. It's like PMI's version of a Shutter that isn't dumb
. That, and it actually has a decent reg. However, I believe it's still cheaper to upgrade from an STS or a VTL to the same thing, check that out first.
As for the difference between the STS and the EXT: unless they changed something recently, they are identical. In fact, All the piranhas are more-or-less identical internally. The only difference that might still be around is the lower end ones (VTL, BTL) had non-venturi bolts (this is not a BAD thing), that were steel I believe(instead of anodized aluminum).