I have a 16 Fist and am leaning towards a BRZ come spring.
I find that the cheap interior (don't like the placement of the little nav screen in the stack), the interior design generally, the semi-unpleasant Recaros and the open-diff FWD hatch are starting to grate on me, while a BRZ - while anemic, is a classic sports coupe with LSD/RWD. I was really hoping the next FiST was coming here - if it included a LSD, i would have gotten it. But it's not, and the FoST is not competitive IMO.
I used to have a Miata, so small and light cars are something i prefer. At least the BRZ can be made to sound nice and you can eliminate the torque dip that makes it annoying (per reviewers). The post '17 models have been revised nicely and they ride substantially better than pre-17s (per reviewers). Looks like for 18 there are minor updates (better head unit on the limited the main one) so you could look to pick up a 17 on the lot at a better discount.
It's a tough call. I love how my FiST drives. When driven vigorously it is joyful. I have kept up with some much more expensive metal. I also was dead even with 3x FOSTs up till triple digits (and left one behind on some curvy roads) who told me he was stage 3. I love so much about the car, the practicality of the hatch is better than a BRZ - but my RWD snobbishness keeps coming back - and the BRZ is far more practical than a Miata/Fiata - otherwise I have been seriously considering a Fiat 124 Abarth to get a small turbo car.
BRZ looks nice, is a pure RWD with torsen LSD sports car, and at least can be modded to get a nice boxer rumble (UEL header) or a nice tuned Porsche like boxer snarl (EQ header). Seems like the header + tune guys are seeing around 220hp/165tq or more (crank). Sucks that there is no perfect little car - a BRZ with the FiST's engine? Done and done... but basically you gotta decide if the turbo torque and the small 4-door hatch practicality are more important to you than classic RWD/LSD driving characteristics, along with the panache coolness of a handsome small coupe compared to a small bulbous hatch.
You can't go wrong either way. Final thought if you are a two-car guy the BRZ is the better garage toy alongside some sort of DD, whereas the FiST is a great sporty DD.
Keep in mind i am a chubby middle aged man - so my feelings about the FiST come from that perspective and ownership of many somewhat nicer cars. I originally bought my FiST last year because i moved and had a 100 mile commute, and my GF works for Ford so I wanted to stick to Ford's stable.