I love my fitbit.
I've got the Fitbit Flex, and the charge can easily last for at least 7 days, possibly a couple of days longer (I forget how often I have to charge it!)
I hardly notice I'm wearing it, and I wear it to bed too to track my sleep and don't notice it at all anymore.
The more expensive ones will track your heart rate too (the charge HR), but for me the Flex was enough as I'm not a hugely active person but it's useful to have
I haven't tried to the Apple Watch, but that's mostly because I can't afford to get one yet
I know several people who have them and track their fitness through the Apple Watch and love them.
Am I right in assuming that the Apple Watch uses the Health app to track everything? That app looks really handy!
So I can't offer an opinion on the Apple Watch, but I do love my Fitbit