Welcome to the Club!
Do you see yourself as a guy who will modify the WRX, if you buy it? My opinion is that if the answer is "yes", you should go ahead and pay the extra for the STi. It will immediately deliver a level of performance that would cost you dearly to achieve with the WRX. (I'm talking the whole package, here, not just straightline acceleration)
If you just want a fun car that you can enjoy, and have prudent financial reasons for opting for the lower payment, then by all means get the WRX. It's a great car for the money.