MongoDB Vs Postgres
NoSQL supports unstructured data, as simple as that.
The same can be achieved by storing json strings in Postgres. Postgres gives you strict rules and foreign keys and holds everything tight unlike Mongo which basically lets you do whatever you want to do - this is a boon sometimes, but it can also be a giant pain in the back side as well occasionally. So, why not use Postgres and enforce some strictness on your DB?
Do let know me why you picked MongoDB over Postgres (Or NOSQL over SQL)