www vs Non-www
The www vs non-www choice determines whether your website is accessible at www.example.com or example.com. Both are valid; the important thing is to choose one as [[canonical-url|canonical]] and redirect the other with a [[301-redirect|301 redirect]].
Example
Most modern websites use the non-www version (example.com) as canonical, redirecting www.example.com to it with a 301.