The Fitbit Sense battery will last up to 6 days.
The Apple Watch 6 Series battery will last up to 18 hours.
Case. Closed. A fitness watch with a battery that lasts a maximum of 18 hours is almost an oxymoron. I suppose Apple’s argument is that a person needs to sleep, so they can charge it then. However, active people, which I assume is who they are targeting, are often on multiple day hikes/bikes/travel, etc, and charging your watch every day is inconvenient. An 18 hour (max) battery life will interfere with tracking your sleep and other metrics because you will forget to charge it (it happens). The Apple Watch 6 Series sure does look pretty though!
Winner: Hands down, Fitbit Sense
You can buy the Fitbit Sense here.
You can buy the Apple Watch 6 Series here.
(Those are direct links to the manufacturer, they are NOT affiliate links)