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

Launch a 2.5 site if you want the most stable that Joomla has to offer and/or possibly if you plan on using many extensions. Launch a 3.0 site if all the extensions that you plan to use are Joomla 3.0 compatible. Only some template providers or the extension vendors will be ready for Joomla 3.0 at launching, however most will not be ready until days, weeks or months after.