In terms of features, performance and smoothness in the long run, stability(lesser bugs), user experience which is better? I have used OxygenOS 12.1 but never used OneUI...
I actually stopped using other launchers in Android and I'm keeping juste OneUI. It quickly restores your preferences from your Samsung cloud backup when you get a new phone. It's not as smooth as what Apple does with the IPhone but still saves a lot of work.
Once you get used to it, the fate of continuing to use it out of inertia awaits. Fingers don't forget the UI. In that sense, I think that an excellent UI is one that you can use without any discomfort.