Unless you keep them stock, it's worth it in the end to go STI. I have a 2008 wrx that now has a built trans, rotated setup (tuned by Dan), and makes 380whp because of obvious block limitations.
The transmission however...is where the money is worth it.
Wrx transmission breaks at...yes.
STI transmission takes a beating.
My built 5-speed cost me $6,000ish...and that doesn't cover diffs.
In the end I have some regrets over the 5 speed wrx. That's my justification.