+1 for Plantuml. It has a great ecosystem, but there are certainly tradeoffs vs using mermaidjs, because of their different compilation models. Mermaidjs seems to look nicer out of the box as well, but usually that's not the point for a sequence diagram.
I'd use whichever works for your ecosystem and diagram lifecycle. The syntax is similar enough that I think the diagram creation constraints are often a bigger difference.
I'd use whichever works for your ecosystem and diagram lifecycle. The syntax is similar enough that I think the diagram creation constraints are often a bigger difference.