As your data complexity and relationships grow, you will end up using SQL, just that it will be in your NodeJS function calls. (Aggregating, Joins, etc) If you

As your data complexity and relationships grow, you will end up using SQL, just that it will be in your NodeJS function calls. (Aggregating, Joins, etc) If you have highly un-related pieces of data, Mongo should be fine. But the finer point is that you can store and retrieve JSON documents very efficiently with PostgreSQL too. So, as an architect, It makes sense for me to choose PgSQL, because down the line, I have features such as relational data available without having to switch-over to other databases.