They serve completely different purposes and are not mutually exclusive. The main takeaways from the ublue stuff is how they build immutable OS images and focus on container-based development workflows. Omarchy is all about ergonomic desktop configuration. They are both cool projects.

You could easily replicate the container-based development workflows on Omarchy with distrobox even if you won't be using an immutable OS image. With more effort, you could replicate the Omarchy desktop configuration on a ublue image. Someone already ported Omarchy to nixos. I think we will continue to see a lot going on with immutable, reproducible operating systems and Omarchy will kickstart some cool stuff with desktop configuration.