Jordan Braiuka compares two models for scaling:
We often get questions from customers about the best way to add capacity to their cluster. Is it better to add nodes, or simply to increase the capacity in their nodes? Unfortunately, the truth is there is no best way—like all complex issues in distributed systems, there are benefits and drawbacks to each scaling approach.
While each of our highly distributed systems (Apache Cassandra, Apache Kafka, etc.) have slightly different implementations of scaling, the concepts remain consistent across most distributed systems.
Click through for a comparison between the two approaches. As the article indicates, both are meaningful strategies and your choice might come down to a combination of the technology stack and the problem at hand.