Went from a GC WRX to a GC STI. I even have a NA 2.2 GC wagon. Although they are not the same cars that you're looking at, they are the same in the sense that you get so much more with STIs; seats, brakes, springs, hp, torque, dccd... all goodies that are highly sought after by Subaru owners who don't have STIs.
They are all tons of fun to drive, but it's ultimately down to what you want the most.
I can tell you right now that if you opt for the WRX, every time you see an STI you will feel nothing but regret. Do yourself a favor and get an STI (

) so you know what it's like to own one.