Comment by mikeadev

Great question. They are similar concepts in that they both allow you to define a C4 model and generate diagrams based on that model. The key difference is simply that PlantUML (and Structurizr) allow you to define the model using markup or code, whereas Carbide uses a graphical interface. I know some people prefer the code or markup approach, and I know others that prefer a simple graphical interface. I think it comes down to personal preference really, but in the end they accomplish the same thing, which is model-first diagramming.