Moving 50 km each week to get the maximun reward is not really a problem for most people . The only case i could see a fitbit being superior is if you struggle with battery life in your phone and need the fitbit do the tracking. I had one years ago but got bored of it after a year, and it was not very stylish at least back then
GamePress
Advantage to a fitbit?
https://pokemongohub.net/post/guide/adventure-sync-in-pokemon-go-everything-you-need-to-know/
According to this article, it's possible to have your data indirectly imported from a fitbit to Google fit.... But is there any real advantage to that? Can anyone tell me if having a fitbit would provide better rewards than just keeping my phone on in my pocket?
