Joomla 1.5 is not supported by the Joomla project for quite some time now. You still can continue to use Joomla 1.5, but (security) updates will no longer be released. Furthermore, both Joomla 2 and Joomla 3 have many new features and improvements. Perhaps you recognize it: once you use Joomla 2 or Joomla 3, managing a Joomla 1.5 website really feels old-fashioned and you notice how greatly Joomla actually improved.
