PlantUML and Mermaid are both Diagrams as Code solutions aimed at providing ways to create UML (mostly) diagrams without taking your hands off the keyboard. They cover a lot of the same ground as each other, and have only really a few points of distinction separating them. Choosing between them comes down to:

Any specific diagrams you need that aren’t supported by both
Your desired deployment environment
Syntactic preferences

Diagram Coverage

Graph Type	PlantUML	Mermaid
‣	✅	✅
‣	✅	✅
‣	✅	✅
‣	✅	✅
‣	✅	✅
Pie Chart	❌	✅
‣	✅	✅
Timing Diagram	✅	❌
‣	✅	❌
‣	✅	❌
‣ Views	✅	❌
‣	❌	✅

Deployment

Both PlantUML and Mermaid offer online diagram generators. Currently Mermaid's is by far and away better than PlantUMLs by any metric.

Online FlowChart & Diagrams Editor - Mermaid Live Editor

PlantUML Web Server

A major difference between the two platforms is that PlantUML expects to be installed locally and used to create diagrams that are saved locally, whereas Mermaid expects you’ll be using it in the browser to render graphs to a webpage.

Installing PlantUML involves downloading & installing:

Java
Graphviz (which handles layouts)
PlantUML

Interestingly it looks like PlantUML is also available via NPM - it’s just not officially supported.

Mermaid’s intended application is via inclusion as a <script> tag using a CDN, or through local install using NPM... both of which are much easier. You could simply copy/paste the below into a text file on your computer, save it with a ‘.html’ suffix, then open it in your browser to see your diagram.

<html>
        <script src="<https://cdn.jsdelivr.net/npm/mermaid/dist/mermaid.min.js>"></script>
        <body>
            Here is a mermaid diagram:
            <div class="mermaid">
                graph TD 
                A[Client] --> B[Load Balancer] 
                B --> C[Server01] 
                B --> D[Server02]
            </div>
            <script>
                mermaid.initialize({ startOnLoad: true });
            </script>
    </body>
</html>

Both Mermaid and PlantUML enjoy some level of native application support. You can utilize them to create diagrams with minimal configuration (and sometimes no configuration) in the following environments.

Mermaid Application Support

Productivity Apps:

Notion

Confluence*

Code Versioning Tools:

GitLab

GitHub - pledged support soon

Code Editors:

VS Code*

Atom*

Sublime*

VIM*

Generated Documentation:

Sphinx*

Typedoc*

PlantUML Application Support

Productivity Apps:

Confluence*

Word*

Code Versioning Tools:

GitBucket*

GitLab

Code Editors:

VS Code*

Atom*

Eclipse*

IntelliJ IDEA*

Generated Documentation:

Swagger API*

Typedoc*

Asterisked items (*) require plugins and/or configuration. Also both lists are partial. See Mermaid’s list and PlantUMLs list.

Syntax Comparison

They aren’t grossly different, but on the whole PlantUML is more descriptive, whereas Mermaid is more graphically representative. Here are a couple of bite-sized examples of the input syntax and output graphics for your consideration. These were created using the online tools found on each of their respective home pages (Mermaid.live and http://www.plantuml.com/plantuml/uml).