The verdict really depends on the user. According to Tech Radar, if you're an iPhone user, you might as well get an Apple watch. You will be able to connect your devices and have exclusive access to Siri and other iOS. For Android users, a Fitbit will better complement your phone. If you are purchasing a watch with exercise as the primary use, the Fitbit edges out the Apple Watch. The Fitbit has functions to track your workout and health stats, coming in at a cheaper price than its competitor.
Although the hardware might be cheaper, you'll still have to fork over some change to unlock all the benefits of Fitbit's premium membership. As Living Etc noted, Apple Watches don't require a premium membership. Plus Apple's device is more durable and customizable. However, if you're willing to splurge on a premiere Fitbit membership, it will deliver greater fitness features than Apple.
