STI can fit 4 people
STI can fit 4 people comfortably, the Corvette can fit 2 people.
STI doesn't risk scraping over speed bumps or moderate approach angles, the Corvette does.
STI is AWD and performs well in all conditions, the Corvette, being RWD would be useless in the snow and still scary in the rain unless driven super carefully.
So if you like to drive on roads that aren't perfectly smooth, live in an area that has 4 seasons, or need to be able to transport more than one person other than yourself, the STI makes more sense, even at the S209 price tag.
The Corvette performs better in a straight line and on a track. The STI performs better literally everywhere else. As far as "performance value" goes, there is much more to performance than simply how powerful a car is or how fast it is. These two are in completely different classes and shouldn't be compared. Personally, I don't like having to turn around and find another way if the paved road I'm driving on turns to dirt. I'd go with the STI every time.