PlantUML is more feature rich, but Mermaid has a distinct advantage: native support for embedding in Markdown documents on both GitHub and GitLab. With PlantUML you either have to give up private repos (proxy renderings) or pre-generate images, which can get out of sync with the source diagram text.

If GitHub/GitLab supported PlantUML out of the box, I'd use it. It doesn't, so I use Mermaid. "Worse is better" in action.