MariaDB vs MySQL for Drupal 6?
CAVEAT : This commentary is solely based on MySQL not Drupal.

I have personally worked with Percona Server and MySQL, not with MariaDB as of yet. Percona provides support for MariaDB, Drizzle, Amazon RDS, and other MySQL products.

I learned at Percona Live NYC that Percona gets the latest version of MySQL and injects 30,000 lines of C/C++ that is unique to its Performance Enhancements. MySQL (eh, Oracle) tries to keep up with its own enhancements of InnoDB.

Unless your Drupal website is very heavily trafficked, there is no decent performance difference you can feel or see. However, if you do have high traffic and you want to compare MySQL, Percona, and MariaDB, I have posted an article in the DBA StackExchange on how to go about doing this.