I like to think of it this way: left of the "/" is for systems/operations, right of the "/" is for code/content/development. So if you're going to put somethi

I like to think of it this way: left of the "/" is for systems/operations, right of the "/" is for code/content/development. So if you're going to put something on a separate subdomain it should be for mechanical reasons, such as using a different cdn or a different hosting stack. Think of it from a workflow perspective. To add /blog to your site goes through your existing code publishing workflow. To add blog.domain.com to your site means involving server configuration or a third party web interface, etc.