gb270 is a lovely car but when i bought my sti i looked at getting a wrx and adding a few prodrive options to improve the weak areas and suddenly i wasnt far off the price of an sti and still didnt have the 6 speed heavy duty gearbox,wider track, thicker driveshafts,dccd and active diffs,nicer interior etc
if you can stretch to it an rb320 is an awesome machine, i have heard a few horror stories regarding the 2.5 engines and oil consumption/starvation so a warranty could be worth it or stick to a 2.0l sti
the running costs between a wrx and sti arent that different as long as you shop around for prices- sti brembo brake pads are about 200 from a dealer,can be found for about 70 oe spec,90 uprated mintex, servicing is every 10k miles or year on sti's -oil and filter change from a specialist keeping the warranty intact is about 140, fuel consumption is not something to think about with any of them lol
on any of the above do watch out for knocking rear shocks which is the one common fault with them
but whichever you go for you wont regret it