Fitbit has a range of devices for fitness tracking. Prices range from $60 to around $250. I own a HR and really like the device. The best part is that the battery life lasts for about a week. And it does charge fully in around an hour or two which I can easily manage when I am at a desk. I mostly try to wear the device at night as well since it also tracks sleep. So the quick charge is a bonus compared to leaving it to charge all night. The phone application is pretty well designed as well. Their customer support is very responsive as well.
A major reason to use a fitbit is their gamification system. You can see your own trends but also those of friends and family. You mostly compete on number of steps taken on a daily or weekly basis. Certain milestones earn other fun virtual badges. All that keeps you interested in continuing to wear the device and track yourself.
Apple watch on the other hand is a generic extension for an iphone. Watch versions of applications allow you to have some interaction with your watch rather than the phone. However this device is expensive at a starting price of $350. They also have a relatively poor battery life requiring a recharge everyday.
In the long term I believe the Apple watch will gain over fitbit. Fitbit’s current game system does get boring after a while. Once you have established a baseline performance measurement, getting a step count is not necessarily that fun. Then come the other useful aspects of the devices like notifications. Fitbit has limited call, text and calendar alerts. A watch has much richer notifications. You can also reply via messages. Other useful apps are Maps and Camera remote. The usefulness will only increase since more apps will be developed. Apple will improve battery life and price will matter less for the premium market. Apple does really know how to build on its existing ecosystem devices.
Pokemon Go is the currently the most popular game on phones. With pokemon, eggs, items, teams, location, badges and so many other game systems they have millions of people hooked and can continue providing new experiences rather easily. They are getting many people to get out and walk around; although in zombie state.
A watch version of the game is going to be a massive killer-app. This is something that fitbit will not be able to get and then pure fitness is not going to be enough.
Update Sept 7th: The watch killer app is here