I can't compare Porto Alegre since I have never gone there except as a waypoint when visiting the extended family, and I have lived in Florianópolis my whole life, so take it with a grain of salt. Personally I like living here, since it's basically a very provincian capital, so you kind of get the best of both worlds: fast internet (very important for us IT folks), decent infrastructure, urban conveniences and calm living.
Housing is relatively expensive on the island, you could rent a house elsewhere for the same amount as a small 2-bedroom apartment on the island.
It is a rather safe city in terms of violent crime, which from what I understand will be a welcome change from PA. Especially on the smaller neighbourhoods.
One interesting thing about Florianópolis is that it works more like 3~5 separate boroughs: Continent, Downtown, North, East, and South, ranked by urbanization. So your experience with the city will vary a bunch depending on where you live.
I have lived most of my life in the South, which feels like a much smaller city, and has the slower pace of life to match, but it means that if you need to go to the bank or refuel you car you have to go almost all the way to Downtown. The upside is that it has a good "nice neighbourhood"/"cost of living" ratio. I currently live in an apartment in Açores (not the Portuguese archipelago!) and it is a lovely small planned-ish neighbourhood which is as far south as a good internet link goes.
If you are a more bohemian person or with an active social life (pandemic notwithstanding) you might be interested in the East, which already has some conveniences like banks and gas stations (not sure why I'm fixated on those lol I don't even have a driver's license) and plenty of good eateries and bars, while still being a calm neighbourhood in the residential areas. I spent a couple of years in Lagoa da Conceição, and despite being a thin-walled kitchenette behind the bus terminal, it was surprisingly chill. Lots of congestion if you're a car user, though.
If you are a native city-dweller and can't imagine going without McDonalds and going to the movies all the time, then you'll want Downtown, or Continent if you're not particular about quickly reaching the rest of the island (there is also lots of congestion at peak hours on the bridges leading in).
I can't say much about the North either since I haven't lived there, but it's generally a big urban sprawl, with lots of foreigners/tourists during the summer season, which means you'll probably have to brush up on your Spanish to do basic human interaction, as well as the general stereotypical infrastructure chaos caused by a sudden flood of temporary residents (supermarket queues, loud neighbours, etc)
That's all to say, if you want to routinely move between these mentioned regions, you'll need a car. Public transport is actually pretty good, with plentiful buses and a very reasonable business model (a bus fare includes transfers between lines at a terminal, as well as distinct lines in the same hour), but that gets old fast. But given the very segregated "boroughs", the only way between them is the road, either on your own car, a ride-sharing app, or bus.
That's basically it as far as general moving advice goes, if there are any specific facts you'd like let me know.