Why should I use InnoDB and MySql instead of XtraDB and MariaDB?
Performance enhancement may be possible without major upgrade to the next version of MySQL or a Migration to MariaDB.

MySQL is the default database by which everyone else creates forks or introduces new things


Different Storage Engines


TokuDB
PBXT
ScaleDB
and Others

Improved Storage Engines


XtraDB over InnoDB
TokuDB over InnoDB

New Features not available in Oracle's MySQL


Multisource Replication
Progress Meter on Commands : Is there a progress indicator for OPTIMIZE TABLE progress?



To be honest, some people upgrade to MariaDB for nontechnical reasons


It's the latest buzzword in the MySQL Universe in years
Redhat want to include MariaDB instead of MySQL in future releases
Monty Widenus really hates MySQL in the hands of Oracle
Hoping for Performance Improvements across the board just doing the upgrade (no due diligence)


Nov 24, 2011 : Why mysql 5.5 slower than 5.1 (linux,using mysqlslap)
Oct 05, 2011 : Query runs a long time in some newer MySQL versions



EPILOGUE

Before deciding on moving over to MariaDB (or any other MySQL fork), you need to be sure you understand the features, improvements, and the configuration settings, and you actually follow through on the settings to make those improvements come to life. See my post How do I properly perform a MySQL bake-off?

In light of all this


If you can enhance performance without an upgrade, you save yourself needless work
If you really need AND UNDERSTAND features, then upgrade
If you benchmark MySQL and MariaDB on the same level playing field (same configurations), decide on which product based on the significant performance improvements (or lack of improvements).