You want people to be able to get to your website whether or not they type “www” before the domain. Both www and non-www versions of a website should redirect to one preferred version of your site. For example, if you’ve set “www.mysite.com” as the preferred version of your site, users should arrive at “www.mysite.com” even if they type “mysite.com” or “http://mysite.com” into their browser.
Why is this important? If your site doesn’t redirect to the one preferred version of the site, Google actually recognizes this as two separate sites. This can cause issues for SEO as Google would recognize this as duplicate content, which can cause search engine ranking positions to be low.
At this time, whether you choose the www or non-www version of the site as the preferred version doesn’t seem to have much impact on SEO.