If your domain is www.example.org, you can host your static components on static.example.org. However, if you’ve already set cookies on the top-level domain example.org as opposed to www.example.org, then all the requests to static.example.org will include those cookies. In this case, you can buy a whole new domain, host your static components there, and keep this domain cookie-free. Yahoo! uses yimg.com, YouTube uses ytimg.com, Amazon uses images-amazon.com and so on.
As it turns out, there are other performance gains to be had from hosting your static resources on a separate domain as well. So you might as well save yourself the $10 per year and use www.
Speaking of cookies, if you intend to grow your website (and who doesn’t?), then you may someday want to begin using subdomains, either for services you want to offer, micro-sites, or some other purpose. Using the naked version of your domain doesn’t preclude you from creating subdomains, but it may cause scoping issues when it comes to cookie usage. I won’t go into the technical details, but you can read more about it at the activist site Yes-www.org.
There may also come a time when you want to use some third party services for improved site performance, like Heroku or Akamai. Both of those services ask that you not use your naked domain as your preferred end point because you might run into DNS issues. As Yes-www.org explains:
[Heroku, for example,] wants to be able to update DNS records in case it needs to redirect traffic from a failing server to a healthy server. This is set up using DNS CNAME records, and the naked domain cannot have a CNAME record.
Search Engine Optimization
SEO appears to have no preference for one or the other, so long as you’re not duplicating content on both. You should always 301 redirect from one to the other, regardless of which you pick.
If you’re now convinced to use www, but you’re already using the naked version of your domain, don’t sweat. In my professional opinion, nothing on this list is critical enough to warrant you to go out of your way to change things up. But if you’re building a new site on a brand new domain, now is the time to make the www choice.
If you’re aware of any other reasons to choose www over the non-www version of a domain name, please share!