The short version
Two subscription-free rings. The Ultrahuman Ring Pro is cross-platform with a ~15-day battery; the Samsung Galaxy Ring is a comfortable, cheaper ring that shines inside the Samsung ecosystem.
When the Ultrahuman Ring Pro is the right pick
You want the longest battery and cross-platform support, and you don’t want to be tied to one phone brand.
When the Samsung Galaxy Ring is the right pick
You own a Samsung Galaxy phone, want tight Samsung Health integration, and prefer a cheaper, more established ring.
Also worth comparing
For the accuracy flagship, see Ring Pro vs Oura Ring 5; for the value option, RingConn Gen 2. Full field: best HRV trackers.