They’re basically the same featurewise - they are both just wrappers for the same rest endpoints so from the point of view of how the communicate to the Camunda engine they’re isn’t a difference
So choosing would would only depend on the how you prefer the features of the individual languages. I’ve used both and they’re both pretty good
Is the REST API Open API compatible so that GraphQL can be generated for it?
Mostly, we have the rest api described using OpenAPI, but it’s not entirely complete. It’ll likely be complete some time in the next 4 to 6 months
In nodejs client there is pollInterval parameter, in java backoff stategy. It is different ways to check if tasks are available
You can also just set up a ProcessHandler using the Camunda Library and let that take care of it. It will get notified when a task that it is waiting for shows up.
OpenAPI compliance is important because then GraphQL can be generated completely off the Rest API or so I have heard.