MariaDB is a fork of MySQL. In my opinion, MariaDB is a better piece of technology. However, MySQL has a larger user base and better managed service offerings.
It allows you to change storage engines to fit particular use cases. For example, it has a columnstore engine that allows MariaDB to function as a decent data warehouse. This is a fantastic feature.
The original author of MySQL is behind MariaDB, so you know there is high level technical talent maintaining the database.
It is fast, safe, reliable, and has a vibrant community.