Both MongoDB and PostgreSQL are database management systems, but they differ in their architecture and intended use cases. MongoDB is a document-oriented database, while PostgreSQL is a relational database.

MongoDB is suitable for handling large amounts of unstructured and semi-structured data, particularly in cases where horizontal scaling is necessary. It is also very flexible and easily scalable.

PostgreSQL, on the other hand, is more suited for handling structured data, particularly in cases where complex queries are needed. It has more robust support for SQL queries and offers powerful consistency controls.

In summary, whether to use MongoDB or PostgreSQL depends on the specific requirements of your project. If you need to store unstructured or semi-structured data and/or require horizontal scaling, MongoDB might be the better fit. If you need to handle structured data with more robust SQL support, PostgreSQL might be more appropriate.