There's absolutely no difference for search engines for using www or non-www version. Different people have different views some say with www Version you have to type four more character. Where as lot have opinion that all user's type first www while opening on browser.Many directories require the WWW in the site location so it is preferable to use the WWW version if you can www is very common, people can get confused if you have a domain without. Don't forget, in your browser when you type "SEO" in the address bar and hit ctrl+enter, it automatically adds the www along with the rest of the URL. For that reason alone, some suggests www version. You may adopt any version of URL but consistency should be there in internal linking.
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