I've been on a steep tech learning curve in the past 12 months.
I have no knowledge of the fitbit, but I do like my Vivoactive3.
I wanted it mostly to record runs and rides, and for MapRuns (orienteering/navigational stuff).
Map trace, distances, elapsed times etc-it does all those very well.
I found Garmin Connect fairly easy to use, but a lot of stuff to just ignore.
It has a weird thing of recording altitude against time, which produces some interesting profiles.
But it is easily linked to Strava, so you get altitude against distance profiles there.
The HR monitor I would only use for comparisons, not sure how absolutely accurate it is.
The sleep monitor is rubbish. I suspect most are.
Even if you don't turn it on for an "activity" it still records steps etc.
It took me about 2 weeks to put all the stuff I don't want to sleep.
It can link into any 2 of 3 satellite systems, usually quickly, but Gallileo seems less effective recently (brexit??)
It can be set to record data at 1 sec intervals. That picked up 5 short hill reps I did one day that Strava completely missed!
Just one person's experience!