MongoDB is really, really bad. I've never come across another product that was so horrible and yet so widely used.

MongoDB: for when you don't need consistency, availability, or partition tolerance.

There are some really good NoSQL products out there. I seriously think RethinkDB is on par with Postgres. I've also used Cassandra and BerkeleyDB and they're both decent. But unless some core part of your business logic is pathological to implement in SQL (like Reddit's comment trees) you should go with Postgres.