I have worn a Fitbit for 5 1/2 years. Until a month ago my most recent Fitbit was the Versa. Now I wear a Series 4 Apple Watch.
I do understand the attraction of the Fitbit. There are areas where I like the Fitbit better. I feel that the Apple Watch is way offbase with its resting calories. I mean, BMR is surely right, but I think it gives too many calories for what it calls minimal activity. And, active calories are too easy to get. I think the Watch overstates calories by at least a couple of hundred a day. I hope this is alleviated in 5.1.
I also like how sleep tracking is built into the Fitbit. Yes, I use a couple of apps to track sleeping on the Apple Watch but they are not as good in many ways.
A big negative of the apple watch is that you look at stuff on your phone and you can't see things at as detailed a level as a result. You can drill down more into the data in some ways on the Fitbit because I can look at it on my desktop.
And, of course, battery life on the Fitbit is far superior.
That said -- I prefer the Apple Watch. I was working out at the gym today and I only had my watch with me. Being the cellular version I was able to take a call that came in while I left the phone in my car. I listen to podcasts while I work out. I have found that there is a lot I can do on the Watch that make it very useful for me in a way that I can't do those things with my Fitbit.
In a way, if all you want is fitness, then I understand getting a Fitbit. On the other hand, I have found it personally more motivated to close circles on the Watch than I did trying to get X number of steps on the Fitbit.
But, of course, YMMV.