I have created a small eBook to explain the differences between Dialogflow and RASA NLU if you are trying to choose one or the other.
The eBook was based on Dialogflow ES.
Since the introduction of Dialogflow CX, it is fair to say that RASA NLU and Dialogflow have diverged into entirely different directions. That is, they are even less comparable now because of the state machine based approach used in Dialogflow CX. In addition (to the best of my knowledge), most of the new Dialogflow features are only going into CX and not into ES, and RASA has also evolved quite a bit but in different directions.
I also don’t have the bandwidth to keep track of all the changes in these two entirely divergent bot frameworks at the moment. As a result, I don’t consider this eBook a good comparison anymore and I have removed it.