Well they're both basically the same car driving wise, except for springs I hear, but it shouldn't really make a difference to differ you from buying one from the other. The front bumper is the only thing different from the outside as well, so decide which front end you like better. Some like the BRZ better, some like the frs better..
The base models for both cars are different as well, the frs does not come with navigation standard, the BRZ has it on base models, which is part of why they are more expensive
You get more with the BRZ, which is why it's costlier. But you can also add on the other options with the frs..
I would reccomend getting a used BRZ personally. The HID headlight's and navigation is worth it. BRZ if you want world rally blue color as well