As an SEO Manager, I am often asked by new clients, "What version of my domain should I use, the www or the non-www?" This question has a simple answer because in terms of SEO (search engine optimization), it doesn't really matter.
In order to choose between domains, there are a couple of things to consider. A new site that has not yet been indexed will probably want to use the traditional www version. For an older site that may already be ranking for key terms, take a look at which version of the site is already ranking and stick with whichever one is ranking. Take a look at the backlink portfolio for each domain as well to see which has more links built.
Once a domain has been chosen, some house cleaning is in order. Update sitemaps and internal link structure in order to reflect the correct version of the domain. Place a canonical tag or 301 redirect on the version of the domain you choose not to use. Finally, log into webmaster tools in order to set the preferred domain.