I have been a Fitbit user for years and recently switched to Apple Watch. I did just wear both for a week to see how they compared.
The other thing -- and this is why I wore both for a week. My experience is that the Apple Watch overstates total calorie burn. It varied a bit each day but on average it says I burn out 10% more than the Fitbit. Given that I track my food and weight, I know that the Fitbit for me is more accurate.
But -- in general -- activity tracker are good at tracking heart rates and are not very good at estimating calorie burn. I mean, honestly, they are all pretty abysmal. Therefore, it makes sense to not really pay that much attention to calorie burn.
The other big philosophical difference is that Fitbit is step based and makes steps the main goal (you can change it but for most it is the main goal). Apple Watch doesn't really care about steps. It care about activity, regardless of how you get it. Of course, you can get it through step based activities (I mostly do) but Apple Watch move ring doesn't really care.