I was in this dilemma when I bought my Sti last year. Seeing how the performance is so close between the wrx and sti, it was hard to choose. Seeing how most of the comments here just talk about how the new FA20 is better than the outdated EJ25 motor, it doesn't justify the WRX being anywhere close to the Sti.
Having been able to drive a 2015 WRX for a week, I was able to compare the WRX and Sti. Even though the Sti has an outdated engine, it more than makes up With a ton of things.
-The Transmission on the Sti compared to the wrx is night and day. The new WRX has a sloppy cable transmission. Yes you can correct it with a perrin shift stop but it still doesnt feel as crisp as the mechanically linked Sti transmission
-The Steering feel on the Sti has a much better feel due to the hydraulic steering and tighter steering rack compared to the electrically assisted steering on the wrx and wider steering ratio.
-Driver controlled Center Diff
-Sti has brembos... it has brembos
-Sti Badges
With the Sti being a track orientated car, it has some drawbacks if being used to drive around the city
-The suspension on the Sti Is extremely jarring. It can make a small bump on the road feel like you went through a lansdowne mall speedbump at 60kms. This can be fixed however with aftermarket springs. I put a set of RCE yellows on mine and it handles the smaller bumps much better while maintaining a stiff ride around corners.
-The Turbo lag on the Sti is quite noticeable. Most of the power is at the higher end of the powerband. The WRX however with the twin scroll turbo provides usable power at lower area of the powerband making it much more enjoyable to boot around the city.
Personally, the extra money was worth getting an Sti. The steering rack and transmission makes driving an Sti night and day over a WRX. I could be biased though because i used to drive a 04 WRX and always regretted not having an Sti when an Sti would pull up beside me. That was back in the day though when everything in the Sti was better than the WRX