Mostly style for me. The Miata has a modern style. The Fiat has a classic style. I do not care for how the older Miatas have aged and believe that the ND will look similarly dated in a decade. I think the Fiat will age better.
Truthfully, neither car knocks my socks off performance-wise. I think they are nice sporty roadsters, but not serious performance cars. I think they are great at what their built for though and am looking forward to possibly buying one.
I think the FIAT badge has bit more allure in the US as well, perhaps as much out of absence/ignorance as anything else. That probably goes two-fold for the Abarth badge. I imagine this will be the Italian sports car many enthusiasts want once in their lifetime but can't justify an Alpha, Ferrari, Lambo, or Maserati (myself included).