The short version
The strongest no-subscription alternative to Oura. The RingConn Gen 3 brings a long battery, haptic alerts and new health insights for a one-time $349; the Oura Ring 5 is the accuracy-and-app leader at $399 plus a membership.
When the RingConn Gen 3 is the right pick
You refuse a recurring fee, want the longer battery and extras like a silent haptic alarm, and accept slightly-below-Oura accuracy.
When the Oura Ring 5 is the right pick
Accuracy and app depth are the priority, and the membership is acceptable.
Also worth comparing
Against the other premium no-subscription ring, see RingConn Gen 3 vs Ultrahuman Ring Pro. Full field: best HRV trackers.