[jd007]

Typically the forks include many performance tweaks over vanilla and so generally perform better. Here is an old test showing 5.5 differences in performance: http://vbtechsupport.com/657/2/

Percona's fork is also much closer to vanilla (than MariaDB), so there shouldn't be any issues even for existing databases. Unless maybe if the system/cluster set up is very complex so there is additional cost for migration. For new projects though, I can't think of any reason to use vanilla over one of the forks.