I chose the WRX. Zero ragrets. No more needing to worry about ringlands and rod knock. If you were to get an STi I would wait for the new motor. That being said, the WRX vs STi stock vs stock are pretty close due to the WRX hitting peak boost at around 2k rpms. The WRX has more usable torque whereas the STi has more of it higher up in the Rev range (linear).
Only thing I dislike about the WRX is that it has pretty much open diffs whereas the new STi has locking diffs which mean you can do sick skids
Another note: if you're ever plan on going for big power numbers (over 600hp) or are planning to track/auto X the car often; I would get the STi. Otherwise stick with the WRX if all its going to see is public roads.
Edit: I don't think buying an STi for the sake of saying " I have an STi " should be a deciding factor