At Localazy, we have decided for PostgreSQL, because with its JSONB data type, it effectively combines both relational and document store. The relational model is still better in many situations. In combination with ACID transactions and other features, it provides you with something you are not about to get with Mongo. The ability to store JSONB gives you flexibility where you need it. Also, the SQL queries are something great and allow you to unload a lot of logic to the database server.