[stickfigure]

I do a lot of work with both traditional RDBMSes and NoSQL databases.

The main question I would ask is: Is your data schemaless? Often it is - especially when storing what we'd normally call a "document". Heavily polymorphic data is often better stored schemaless. And sometimes you don't necessarily have the schema in advance (common when storing "other people's JSON").

You can store schemaless data in Postgres via the JSONB type, so this isn't necessarily a "Mongo vs Postgres" issue, but more of a general data modeling issue.

As a point of reference, the folks that struggle with schemaless tend to be the ones using Javascript, Ruby, or other type-ambiguous languages. Schemaless is less of a problem in Java and other languages where class structures enforce your schema.