SEO Inquiry: Subdomains vs Subdirectories
SEO professionals have long debated whether subdomains or subdirectories were better; we've known the answer for years (it doesn't matter) but questions still arise.
In an informal test of this question, it appears that the concentration of sites appearing in the search results is somewhat low (your own results, of course, may vary). Using the query "easter gift ideas for adults," Google showed just three of thirty listings with a sub-domain (while Bing just two). Those in the Google index were a community subdomain at babycenter.com, a weblog post from ebay.com, and the answer subdomain at yahoo.com.
These findings likely reveal businesses' preference for subdirectories over subdomains, rather than Google's. This sameline of inquiry was posed to Google's John Mueller in a recent Google Webmaster Central hangout who indicated (confirmed really) that Google does not treat one or the other as superior.
"From our point of view they are essentially equivalent. So you could do lots of things with subdomains and you can do lots of things on subdirectories, it kind of depends on your infrastructure which way is easier for you to handle. So from our point of view this is essentially up to you. If you want to put a blog on your main website in a subdirectory that’s fine, if you want to use a subdomain that’s fine. If you want to use a different domain name, that’s possible too."