As far as I know, both of them could be used with WordPress, but I'd like to know if anyone uses them, and if they really help make our blogs and servers run better than they did with MySQL.
I have personally worked with Percona Server and MySQL, not with MariaDB as of yet.
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 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.