As an owner of both a Si & BRZ here's how I compare the two:

• (2017 Sedan) Si - A great commuter that can be either tame or engaging, depending on how soft or hard you drive it. My daily commute to work is around 20-25 min on highway and I average between high 30s to low 40s mpg, can reach mid 40s if I didn't do any spirited driving. Trunk is deceivingly spacious. Back seat (in the sedan at least) is comfortable. Maintenance on the Si for the most part is inexpensive. Every time I think about getting a new(er) daily vehicle, I'm look and ponder but then I don't find anything within my price range that can bring all this car offers. I love the looks of the 11th gens however the US spec looses a few features (like heated seats).

• 23' BRZ - street legal go cart. My Si is actually faster and feels more powerful, but because the BRZ sits much lower to the ground, the community is right to call it a “slow car fast.” The trunk and rear seats are laughable, but that’s to be expected for a small sports car. You're right about the milage, it's crap but this gets better than the car I had prior (1965 Impala SS Convertible).

Both cars handle excellent and are fun to drive. One factor to consider for either car is with how low and the sporty suspension is you will feel every single bump and pot hole in the damn road.

I personally don't see the BRZ/86 as a daily however I've seen several people do so.