try both the wrx and sti,only way to be sure
but bear in mind whether your looking to keep it standard or tweak it a bit - theres plenty of cases on here of people who bought wrx's and started adding bits only to realise they should have one straight to the sti - i looked at that when i bought mine, 4k cheaper for a wrx but by the time i added a bigger spoiler, bigger brakes, ppp kit i was up to the same price -without the better diffs,stronger 6 speed box,nicer interior,better suspension etc and i was paying to insure a modded car too
whatever you get youl enjoy though