Pretty much old news
Pretty much old news here.
Having driven both, several times, I have come to the same conclusions. The BRZ is tuned for sharper handling, and the FRS/GT-86 is tuned for the younger crowd, the wannabe drifters who think they can drift. Most will collect curbs and telephone poles and drive up the cost of insurance on these cars.
I did the Scion FRS 'First Drive' event at Portland International Raceway, on an auto course set-up. The FRS's were not stock, and were set-up with Enkei 'UL' style wheels, bigger tires, and tuned lowered suspension. Using this set-up they were composed and sharp in the quick turns. So buy what you want and if your want a composed sharp handling FRS, set it up for corners and enjoy.
As far as rear seat room, these cars are true GT's, not hot hatch's, but they will hold 4 race tire/wheels, a floor jack, and a small tool box, and corner scales for track days or auto cross work.
Regarding power... Plenty available for those who can handle it. For those who's only performance efforts are blasting down an on-ramp, buy something else, like a clunker 2013 Mustang.