Postgres is a fantastic relational database but for storage, indexing and querying of JSON documents there is no comparison to MongoDB. MongoDB is built from the ground up as a JSON store (actually a binary encoding of JSON called BSON). I would encourage you to try it out I think you would be surprised by what is in the box.

One of the benefits of using Mongo is its horizontal scalability, not necessarily its ability to store documents.

That is somewhat missing the point of MongoDB. We start with easy to use document storage, querying, indexing, and dynamic schema. We end with horizontal scalability.