I've used both, and the only reason we switched to Flask-restplus was the desire to have auto generated Swagger documentation. In my experience, there are no other noticeable differences. Flask-RESTplus started as a fork of Flask-RESTful, so if you were so inclined, you could read the commit history and see if there are any other noticeable differences.
update When reading this accepted answer, consider that there is Flask-RESTX which is a fork of Flask-RESTPlus that is maintained, as an alternative option.
I am aware of the fact that this answer is probably too late, but it still might be helpful in the future.
According to https://github.com/noirbizarre/flask-restplus/issues/593, the Flask Restplus is most probably dead and abandoned. The last commit was on October 1, 2018. It looks like the project is not being actively maintained anymore.
Therefore, I advise that you use Flask Restful despite the auto generated Swagger docs in Restplus. Since Restplus is not maintained, it is most likely that the implementation will sooner or later go out of date. Rather than switching to Restful once Restplus goes out of date, I think starting off with Restful itself would be the better idea here.
update You should also consider using FastAPI which is a framework that is becoming popular (see google trends). And, differently from Flask: has built-in data validation, supports asynchronous tasks and you also get the Swagger UI.
You stablish the endpoints with decorators like in Flask and it is reportedly faster.