Wait for Apple Watch 5 Reviews
The Apple Watch 5 looks great and it will likely be a great device thanks to multiple iterations and no groundbreaking changes from the Apple Watch Series 4. That said, you won't know everything about it until the Apple Watch 5 reviews arrive.
The first Apple Watch 5 reviews will start up next week ahead of the release from publications that Apple offers up early review units to. These are great for up front reviews of new features and changes.
You may also want to wait and see what longer-term reviews look like and what general users have to say about the performance and reliability once it is in the hands of regular users.
If you are on the fence, or thinking of upgrading from the Apple Watch 3 or 4, hold off and read or watch reviews before you buy.