You have an established site for some time and you just realized, or some SEO consultant told you, Google see your WWW and non-www pages as different pages with same content. So, you shall make 301 redirect on one way or another. but what way to chose?
Make a small analyze first. How many www pages Google indexed from your site? And how many non-www?
Then, analyze inbound links. How many links do you have to www pages? And how many are pointing to non-www pages?
So, if you have more non-www pages indexed and more links to them, make 301 redirect from www to non. If that’s reverse, make a reverse redirection. And big SEO issue is solved.
Simple as that. 🙂