The rule that I use: - everything user-generated that you have little control over (for example, forums or blogs with badly moderated comment section) goes to

The rule that I use: - everything user-generated that you have little control over (for example, forums or blogs with badly moderated comment section) goes to subdomains. - everything you control goes to subfolders. Thus, if you can keep your blog free of spam, it should go to subfolder.