I wear both, lol. I have the Fitbit One, which is a clip. I lost weight using a Fitbit way back before they were popular, like 2010. To this day I still use their app to track my food. It’s very easy and intuitive. Many of my friends have a Fitbit so it’s fun to do challenges.
I got the Apple Watch series 3 with LTE because I am a runner and I never carry my phone with me when I run. In the winter I’m out there in the dark by myself (I’m a woman). So the LTE capability really sold me and it’s been very much worth it for me. In addition to feeling safer, I have a few weeks out of the year where I’m on call for work and must be reachable by phone. I’ve already used the watch to field duty calls! I do like the activity app but not many of my friends have an AW yet so it’s not as fast paced as a Fitbit leaderboard. I do like the Workout app. The GPS is as accurate as my Garmin was and I like that you can choose other activities for it to track.
When my Fitbit dies I probably won’t replace it since it’s now discontinued and I won’t wear two armbands. And I love my AW too much to give it up now!