Garmin watches are designed more for those who do sports and know what they are doing. For example running. And for tracking running data with garmin watch is better way than using mobile app, this kind of watch also keeps you more motivated during the run because of shadow feature and the advantage of garmin shadow is that it is more reliable than app shadow. Also you can extract all the data into you PC and analyze by yourself, which is more important feature for those you are more into professional sports.
As for fitbit is it quite useful for those who sits in the office all day and if you like to track everything. This wearable is quite trustworthy and it shows all the data for the user in a simplified way, so everyone in a glance can understand it. As for using it for sports, garmin will be better choice.
Hope, I answered your question