Or PostgreSQL. I've been hearing good things about it lately.

MariaDB ships with XtraDB, Percona's modified InnoDB engine. If you're planning on using InnoDB (which just about everyone is) the differences are pretty minor.

The big difference is that MariaDB comes with the Aria storage engine, which is a crash safe alternative to MyISAM.

http://kb.askmonty.org/en/about-xtradb/

http://kb.askmonty.org/en/aria-faq/

If you're using mainly InnoDB and want more reliability / performance / less variance in execution time - go for Percona, that's what they're specialising in. Also you can use the company for support when needed.

I don't think Percona sees itself as a fork: it is just MySQL + special Percona patches.