I chose the BRZ, the insurance rates are cheaper than the FRS.
I wanted the leather seats, nav, keyless entry, LED headlights, and push to start, which the FRS don't offer.
Scion offers TRD parts, but in my exp the TRD is just expensive toyota branded horseshit. (They offer STi parts for the BRZ too but you have to have them direct shipped from japan.)
If you want aftermarket parts, buy them from company's that don't manufacture the car itself.
They are the exact same car though, meaning i can buy headers, a full turbo kit, brakes, intercooler, and install them into an FRS or BRZ, because same car
The insurance rates being cheaper on the BRZ, although they are the same car, is due to the volume sold.
There are well over twice as many FRS's on the road then BRZ's (due to an 9 to 1 contract agreement between toyota and subaru... 1brz can be produced per 9frs's produced.)
So more cars = more accidents = higher rates on FRS.
whatever you end up with, you will love it