THe only major reason I can see to go for the WRX is either because you hae driven it and just like it better, or you plan on doing power mods in which case very few cars today can touch the power potential of a WRX. There is simply a sea of simple to install mods that make real nice power. MOst are tried tested and proven at this point. Hell the most diffucult whing is deciding which of the 4 major vendors your going to go with. Vishnu, Turboxs, ecotec or Cobb. Plus there a bajesus more of them out there.
If your thinking about the 3, put power gains out of your mind. Its not going to happen. There is a very good reason that 99% of the people tha turboed there proteges kept them for less then 2 years afterwords. Its not a simple procedure and its just one long ass headache of crap going wrong. Even if its just little stuff. Plus the warrenty is istantly void and unless you have some real talent in your area, you will be on your own to deal with it all. Also, unlike a WRX, the 3 will not make big power with bolt ons. Like all current mazda's its just regualted to strickly by the ecu.