GitHub:

We strongly recommend that you use a www subdomain for these reasons:

  • It gives your GitHub Pages site the benefit of our Content Delivery Network.
  • It is more stable because it is not affected by changes to the IP addresses of GitHub’s servers.
  • Pages will load significantly faster because Denial of Service attack protection can be implemented more efficiently.

Then from yes-www.org:

You should use www because today you have a small web site, and tomorrow you want a big web site.

Two key benefits of WWW are that:

  1. It can be set up with CNAME records which allows for balance loading with dynamic IP addresses.
  2. It keeps cookies created on the primary site inaccessible to subdomains. Access could cause unexpected problems and no clear benefits.

I’ve noticed that the following sites prefer the WWW subdomain for their primary sites: Apple, Google, Facebook, Microsoft, Yahoo, and Strava.

I’ve switched.