Hi, I haven't used one, but my friend is using an Oura.
Her son gifted it to her, afaicr it was around £300, not sure how much the subscription is.
I have to say it's very effective, on a couple of occasions we knew my friend was under stress, she denied it, but the ring said otherwise lol
From my limited knowledge of the ring, I think there's a flaw: it's highly effective in detecting changes in heart rate, sleep patterns, yes, but the app that gives you the readings cannot be shared.
That is, if the ring warns you that you are on the brink of collapse and need medical assistance, the alert is on your own phone only, her son cannot have access to the data from his phone, in order to call an ambulance or to assist as needed.