it doesn't matter whether you use www or non-www. What matters is that you're being consistent (use either www or non-www everywhere) otherwise google considers that you have 2 sites and you might run into duplication, cannibalization, and link juice reduction issues (because some links will point to the www version of the site, while others will point to the non-www version, which means your links will be split between 2 "different" sites, which means that the authority offered by your backlinks will be divided, which ultimately means less / worse rankings for either version of your site)
Just pick whichever version you like and use that one all over the place...