For new sites, the Joomla! 3 series is the preferred series and starting on it avoids an update from Joomla 2.5 later down the road. Starting on the Joomla 3 series for a new site, also provides you with longer backward compatible support (with one-click upgrades) than starting a new site on 2.5. Joomla 2.5 will reach EOL (end of life) 31 Dec 2014.
Sites that are currently on Joomla 2.5 are encouraged to start planning now for an upgrade to 3.3 or greater, unless you want to move over sooner to take advantage of Joomla 3’s new features. We’ve provided a one-click upgrade from Joomla 2.5 to any Joomla 3 version and you just have to make sure that all your extensions and template are compatible before upgrading.