Recs.
Updated
SpecsUpdate
Pros
Pro Great speed
Mysql has always been faster than Postgres but when the creators of MySQL sold it to Oracle and then continued development on a fork called MariaDB, they surpassed Oracle who didn't maintain MySQL for over a year and improved it well beyond what Oracle currently has done.
Pro Flexibility
MariaDB/Mysql has enormous flexibility in that it enables you to swap out engines for your needs, it autoincrements and send back key with INSERTS. It takes into consideration alot of basic concerns that may not be ACID compliant but are more in line with day-to-day business logic and modern api development.
Pro Truly open development community
All development decisions for MariaDB can be reviewed and debated on a public mailing list or in the public bug tracker. Contributing to MariaDB is easy and the patch flow is fully transparent and public. But it's not all about the code contribution either - MariaDB also has very active documentation efforts and other related things that help developers in their day-to-day database administration.