There are more differences than just the turbo between the STi and GT (in fact, I don't even know what turbo the GT has). The STi is rockin' a VF39.
Bottom line: for all out performance, get an STi. For very, very good performance with a healthy does of luxury, get the GT. The GT should be cheaper when all is said and done. Also the 5-speed in it should hold up just fine.