Welcome!
Owning both a 2019 Micra S & 2012 500 sport (both with a manual gearbox)... I have to say each are absolutely fun cars for their own reasons!
You can run regular gas with no issues in a non-turbo 500. It is premium 'recommended' but not necessary. If you get the turbo, premium fuel is absolutely required. In terms of absolute 'smiles per gallon', both the 500 turbo & non-turbo win hands down.
Pros for the Micra: Simple, fun, cheap, more reliable & utilitarian & if you can find aftermarket parts... it's a rare care to modify. It's also the most 'stable' car out of the two. Cons for the Micra: Extremely susceptible to crosswinds, gearing is way off (too long in 1st & too short in 5th).
Pros for the Fiat: Also cheap, insanely fun to drive (Don't get an automatic), 4 wheel discs, charm and surprisingly spacious inside, more fun to drive than Micra, more airbags, chassis feels more solid than Micra, lively engine & better gearing. Cons to the Fiat: Not an ideal primary car... too many small & cheap things that go wrong, has a timing belt, holds less people than Micra but back seats in a Fiat are perfectly fine for two small-ish people, no rear-swaybar in a non-Abarth model.
Would I sell my Micra? Absolutely not! It is the better 'all-arounder' & does it perfectly. In terms of 'pure' driver's enjoyment factor, 500 Turbo or Abarth win hands down.
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View my fuel log 2019 Micra S manual: 7.8 L/100 km ... 36.3 mpg (Imp) ... 12.8 km/L ... 30.2 mpg (US) ...