MongoDB is a distributed database that is built for modern transactional and analytical applications and may be used for rapidly changing, multi-structured data. On the other hand, PostgreSQL is an SQL database that has all of the features that you require from a relational database. If you are unsure of the differences between these systems, on the MongoDB website, you can find an article that compares PostgreSQL and MongoDB.
Choosing one or the other between MongoDB and PostgreSQL may not be your only option – in fact, because each database has different strengths, you may wish to use them side-by-side. If this is your case, then you may need to sync data between them.