I own both a Fitbit Vivo Active 4S and a Garmin VivoSmart 4.
In the daytime, as a watch, I much prefer the Garmin, but at night, as a sleep tracker, which is the main thing I want from these watches, I only trust the data from the Fitbit.
If I'm concerned about actual total sleep times, I do generally need to edit the Fitbit data in the morning. Over the years, my Fitbit watches have fairly consistently confused idle time sitting on a couch watching videos or lying in bed reading on my phone before or after sleeping as sleep. The data edit tools in the app fortunately make it easy to correct this (I'm not someone who falls asleep with the video playing, so I generally know when I turned off the activity at night and when I woke for the morning).
The data during the night, which is what I care about, I've come to trust on Fitbit watches over the years. I can't know if it's precisely accurate, but it's definitely directionally correct (if there's an overall scale factor error or similar from true sleep quality and time numbers I don't really care as the measurements are consistent enough that I can use them and benefit from them).
My Garmin I view as a random number generator as far as sleep tracking. My perception is it's probably a bit better about not misclassifying idle time before and after sleep, but not accurate enough to be actually correct, and I have not been able to find any simple data editing feature in the app to fix the misclassifications, so the wrong data stays wrong.
The Garmin data during the night I have zero confidence in, as anecdotally it feels far less "believable" than what I see reported by my Fitbit.
Edit added: another commenter just mentioned that Garmin uses a learning algorithm that needs about two weeks of data to hit good data quality. I've done a couple periods of sleep tracking with my Garmin, but I'm guessing I've abandoned it due to poor data quality each time at around two weeks. Sounds like I should give it another try and force myself to wait a month or so before making any assessments.