My rough understanding is that Redis is better if you need the in-memory key-value store feature, however I am not sure how that has anything to do with distributing tasks?
Does that mean we should use Redis as a message broker IF we are already using it for something else?
I've used both recently (2017-2018), and they are both super stable with Celery 4. So your choice can be based on the details of your hosting setup.
In the past, I would have recommended RabbitMQ because it was more stable and easier to setup with Celery than Redis, but I don't believe that's true any more.
AWS now has a managed service that is equivalent to RabbitMQ called Amazon MQ, which could reduce the headache of running this as a service in production. Please comment below if you have any experience with this and celery.