PostgreSQL is the newer, standard open source, relational database option for most Linux based servers. Many of MySQL's shortcomings are addressed with PostgreSQL, and it will work well with open source ORM's used by development frameworks like Django without a lot of additional work. Where MySQL can require a significant amount of query optimization to stay responsive, PostgreSQL handles sub-queries much more elegantly, letting your developers work on code instead of architecting a database. Postgres also has a reputation as a much more secure service, although both it and MySQL are very secure when configured correctly.
There is very little advantage to choose MySQL over PostgreSQL. Aside from some extra work during the initial setup, which can be automated with a deployment tool like Ansible, Postgres is superior to MySQL in every way. Unless you are looking for a "NoSQL" solution like MongoDB, it will be a good fit for your project.
Like all relational databases, a good table architecture and schema are required to get long-term performance from your project. Having developers familiar with PostgreSQL is key.