I can’t give you a recommendation which framework you should use. It depends on your use case and other conditions.
However, as far as I can see Spring Data Flow has a slightly different target group (note that I don’t know much about the framework). The description on the project site says
Spring Cloud Data Flow is a toolkit for building data integration and real-time data processing pipelines. Pipelines consist of Spring Boot apps, built using the Spring Cloud Stream or Spring Cloud Task microservice frameworks. […]
In contrast, the Camunda platform is described as
Open source platform for workflow and decision automation […]
In Camunda, you can model your workflows using the BPMN standard which is very powerful and flexible. Also, it makes no restrictions how to implement or integrate your business logic (e.g. micro-services).
Spring Data Flow seems to be more specialized on data integration and processing pipelines with Spring applications.
You can find more information about Camunda in the docs. Also related:
If you have more specific questions then feel free to ask.