PostgreSQL is one of the most powerful databases on the market, providing high levels of ACID and SQL compliance. It’s a Free and Open Source database, built with extension in mind. Its popularity puts it ahead of “giants” like Oracle and SQL server, and it is used by companies like Apple, Instagram, and Spotify. If you are serious about working with data, then this is the database for you.
MongoDB is the leading NoSQL database, and it ranks in the 5th position of all database popularity, just after PostgreSQL. It's also a Free and Open Source database, aimed at people who are building web applications that need to evolve quickly and scale elegantly. Uber, Twitter and Ebay are some of the companies using MongoDB in production.