Battery and cost
The Garmin Vivosmart 5 lasts well past the Fitbit's seven days and needs no subscription. The Fitbit Charge 6 charges more often and locks its best features behind $12.99 a month.
Tracking and GPS
The Fitbit has built-in GPS and more detailed workout tracking. The Garmin leans on your phone for GPS and keeps tracking basic. For data depth the Fitbit leads.
Which fits
Richer tracking and a livelier app point to the Fitbit. Longer battery and no subscription point to the Garmin.