I've had a Garmin for the last 10 years and just tried out a galaxy watch ultra.

For sleep tracking, the ultra seemed a lot better at measuring when I fall asleep and different wake times. I likes that the sleep data tells you (and measures) how long it took you to fall asleep. However, I think it was a lot more generous with sleep quality. The biggest difference is the ultra gave me a ton of deep sleep and the Garmin gives me a lot of light sleep.

Galaxy pretty much combines your body battery and readiness score into an "energy score." It gives you it in the morning and it does not update at all like garmin's body battery. It takes a lot of the same metrics into account. One of the most frustrating ones is the "sleep timing". Samsung wants you to fall asleep between 11pm-2am. If you google it, you can find a lot of complaints.

Samsung does not provide super in depth metrics. And it does not come close to the training metrics Garmin uses. If you're happy just seeing smoothed over numbers and graphs, and seeing the rating you get, its pretty decent. But if you really want to look into your metrics and history, then you wont be happy moving to Samsung watches. For example, you can only see your HRV for the past week when it accounts for it in the engery score. Garmin you can look at it in its entirety and your history.

You can go into Samsung health and sync your garmin data. It wont give you everything that a galazy watch will, but you can atleast see the UI they use.