Not to harp on about the GB270 vs STI thing too much, but here are some of the goodies the STI offers over the WRX/GB270:
6-speed gearbox - the upper gears are close ratio, very useful when you're pressing on through the twisties. The 'box can apparently also handle much more torque than the WRX's - handy if you're ever looking to boost engine power.
DCCD (driver-controlled centre diff) - you can effectively vary F:R torque split from 50:50 to 41:59 (either manually, or by letting the computer do it automatically). Doesn't sound much, but the latter mode gives the car a palpable rear-drive feel - the back-end comes alive, steering becomes sharper, and you can even generate some throttle-on oversteer.
Bigger (Brembo) brakes.
Bigger turbo.
Bigger intercooler.
Smaller steering wheel with a higher-geared steering rack.
Wider track.
Slightly longer wheelbase.
Forged crank and conrods.
Bigger injectors.
STI instrument panel (much nicer than the WRX item).
Tracker (don't think the GB270 has one as standard?).
You might know about all of this already, but I didn't before I bought my first Scoob (WRX). If I'd been better informed, I'm pretty sure I would've gone straight for the STI