I have an Apple Watch, and like it as it synchs to my phone and other Apple devices, meaning that I get texts and other notifications on it, and can just glance at my watch rather than get my phone out to read them.
The health things are fairly reliable - I have checked them against an NHS approved pulse thing (the fingertip ones) and my BP monitor. I like the sleep tracker, and the fact that the app i use notifies me if there has been a change in things like oxygen or overnight heart-rate dip, and as it allows me to log things like eating late or whether I've used an eye mask I can tell what helps me to sleep. I don't bother with the snore monitor as I prefer to have hypnosis or rain noise in my ears when getting to sleep, and you can't record and play at the same time.
My friend has a Fitbit, and it is much more basic - it measures steps, sleep and a couple of other things, but doesn't have the apps that an Apple Watch does. Maybe they have moved on since she got hers, and obviously a lot depends on what you are wanting from one.