Should I launch a Joomla! 2.5 site or a 3.x site?

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Joomla! 2.5 reached EOS (end of support) the 31 Dec 2014. That means that since that day it will recieve no further attention or support from the Joomla! Project. No more maintenance or security releases. So you should not start a new site with Joomla! 2.5, but always Joomla! 3.3 or greater.

Sites that are currently on Joomla! 2.5 are encouraged to start planning now for a migration to 3.3 or greater now. 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 with Joomla! 3.x before upgrading. See Planning for Mini-Migration - Joomla 2.5 to 3.x

See also: Why Migrate and all the related pages.