Apple

First thing in my mind is the battery. My son has a five year old fitbit charge 3 and he says the battery lasts him a full week on one charge. My S6 needs to be charged virtually daily.

The heart rate monitoring in fitbit is a lot better because it's continuous and you get this nice easy to read graph in the app. Apple watches don't read your heart rate every second and the apple health app gives you a bunch of vertical lines which don't give precise data.