I use Autosleep and it appears remarkably accurate. I get much better data from the S4 and S5 than from the S0 I used to use. I have a feeling the S3 might be as good, but earlier series not so much. HRV is one of the bigger differences. S4 and S5 also record noise during your sleep, not sure if S3 does or not.
Fair comment, but if you keep your old watch when you upgrade you can use that for sleep tracking with no battery worries. Given how much complaint there is on here about the worthlessness of used Watches then that makes lots of sense to me. I actually use an S4 Hermes for sleep tracking after buying it used very cheap to get the bands. As my S0 battery was swelling so I couldn't keep using it for sleep tracking it worked out to be a great deal for me.
It will be a WatchOS 7 feature, not a Watch S6 feature I expect.
I guess I'll wait to see what the baked in sleep tracking is like before I decide which to use. Of course I can use both as Autosleep just bases its reports on data its granted access to in the Health data.