The STi interior is a little nicer and the bigger engine is a bonus. I love mine. The evo is awesome too. DustinR on the board has both and STi and an Evo MR. He loves them both, but for differant reasons. The Evo handles more precise than the STi, but the STi feels like it has more power.
Your call of course, but I agree with Nick, both are awesome choices.
Alot of the people that I know that have WRX's and compete with them mod them.....alot, or have changed out to an STi (in some form or another). It is not a bad car, but the alternatives are more fun I think. If you get a WRX you should get a wagon. Those rule.
Or perhaps you could go with the Legacy GT. Those are the new hotness.
Good Luck Phia!