Data aside, neither are ideal. I can’t sleep with the Apple Watch because it needs to be charged while I sleep to get through the day, and the Fenix is too heavy for my wrist at night. I use an old Mi Band, which tracks approximate data that suits me.
According to reviewers, Mi Bands and generally all of Xiaomi's devices are fairly bad at sleep tracking. So much so that it's almost pointless to use them for that.
I know it’s fully anecdotal, but my Xiaomi/Huami (former Xiaomi sub-brand) devices have matched up sleep duration quite well with what I feel in the morning - and I’m quite sensitive to less sleep. I think it’s quite good for sleep duration, but I don’t rely on REM duration etc as I don’t have much use for it. When it predicts a good sleep score I’ve definitely slept well, though sometimes it gives a lesser score even when I don’t appear to have slept poorly.
(I’ve used a Mi Band 1, an Amazfit Bip, and currently an Amazfit Band 7.)
FWIW, I charge my watch while taking a shower before bed and in the morning, and that seems to be enough to get me through the day and through the night.
