so the only advantage is better brakes and better AWD?
well.. i guess you could buy a big brake kit for your WRX for less than the cost of an STI upgrade..
The STi also has a quicker (hydraulic vs the wrx's electric) steering rack, different suspension and chassis tuning, BBS wheels (on launch and limited trims), LED lowbeams, leather/alcantara heated seats, heated mirrors, dual zone climate control, and optional keyless start.
Performance wise, the STi has always had a little bit more power, better AWD/diffs, better trans, and better brakes as the main selling points. That is nothing new, the only difference is the transmission is the same now. You could buy the big brake kit aftermarket, but you will be close to the price difference if you do both the front/back brakes and you won't have the better diffs.
Engine wise, only time will tell if they put a different tune on the STi or addressed any of it's issues. The WRX is unproven in the form it will be appearing. I do think it's a shame they didn't put a new motor in the STi though.
http://www.caranddriver.com/news/2015-subaru-wrx-sti-photos-and-info-news