The WRX is a street car that can be taken to the track. The Sti is a track car that can be driven on the street.
THIS THIS THIS!
Here's my .02:
Buy the WRX premium. I had a 13 WRX Sedan that I really liked. The driveability was great and I loved the car. It didn't get as good of MPG as the new WRX, but it was still a good daily driver. However, like you, I always had a fetish for the STI. I get the neck break thing. As a result, I traded in my paid off WRX for a 2015 STI. Do I love it? YES... But I wish I would have saved money and kept my WRX. The STI is nice, but it is a car built for the track and it will feel like a car built for the track. Is that really cool? YES... at first. I do love it most of the times, but there are occasions where I think to myself... damn this car is rough. Save your money and buy the WRX. It's enough car to have a blast in, without the annoyances of a car that only drives at it's smoothest when you're pushing it and something that doesn't feel like it's riding on wagon wheels.
As for the wing... Some people will say, "It's your car and do what you want." But since you asked, here's an additional .02 for you. Leave the ****ing park bench on the car it belongs on. You will have several things going against you. #1. You'll be a 'poser.' #2. You're going to have people constantly asking if it's a STI, in which case you will have to semi-shamefully say No, it's not. Trust me on this, since I got that question constantly when I owned my WRX and it DIDN'T have a massive wing. #3. The thing attracts so much attention. I know as a younger guy you're going to think that's a good thing BUT IT'S NOT. The people who will appreciate your car will see it coming down the street WITH or WITHOUT the wing. On the flip side, most everyone sees the huge wing on the STI and you get several responses. The looks of "Oh, another dumbass kid." or "WTF is that???" You'll also get endless amounts of ricer fly bys and anything from actual fast cars to pieces of **** will pull up to you and challenge you. You can make your WRX look just as sharp without the park bench. The majority of the people who will appreciate it will be the few who think they're looking at a STI. #4. At your age, most chicks don't dig it... just sayin. (Unless your my wife, who is not a car person, BUT AGAIN, only likes it because she thinks the wing is part of what makes the STI what it is.)
Cliffs: Buy the WRX. Don't waste your time, money, or common sense on the wing. The only other reason I could see buying a STI over the WRX is if you're one of those people like me who loves Subaru's for the nostalgic unequal length header "boxer rumble." Yes, I knowingly purchased an inferior motor primarily based on the sound that I love so much. (shoulda kept the 13 WRX)