There was already a thread about this...albeit in the Track/Autocross section so the discussion was focused mainly on that...
I used to have a mildly modded '94 Miata. Although I loved the Miata, I don't regret going to the FR-S at all. The only thing I really miss is cruising through the city with the top down.
The main reason I switched was simply because I got a new job and suddenly had a lengthy commute. I loved my Miata but 70 miles a day (35 miles/45 mins each way) was killing me. I thought about all types of tin-topped cars but I've been in love with the FT86 since I first laid eyes on the concept.
Miatas are EXTREMELY slow. We're talking 9-seconds-to-60 slow. The FR-S weighs more, gets better fuel mileage, and makes it to 60
three seconds faster. Although I've driven many much faster cars, the FR-S is plenty fast for me, especially on the horrible stock tires. Hell, the Miata was
usually fast enough for me anyway, except when it came to passing on the freeway.