There is a lot of confusion on which one is better – sub-domain or sub-folder. Google is not willing to specifically tell either being more relevant in terms of search engine preference.
Google’s Matt Cutts, back in the end of 2007, wrote an article about subdomains and subdirectories, in which he told that Google prefers subfolders to subdomain when the site is about closely related products:
My personal preference on subdomains vs. subdirectories is that I usually prefer the convenience of subdirectories for most of my content. A subdomain can be useful to separate out content that is completely different. Google uses subdomains for distinct products such news.google.com or maps.google.com, for example. If you’re a newer webmaster or SEO, I’d recommend using subdirectories until you start to feel pretty confident with the architecture of your site. At that point, you’ll be better equipped to make the right decision for your own site.
Although, Matt says subdomains or directories does not make a huge difference. Having different subdomains for closely related stuffs would fragment the site in search engine to believe they are different products. For example, if you have a site about movies in "moviesite.com", and if you make "actors.moviesite.com", "actress.moviesite.com" Google might think they are three different sites.
There is a chance that "actors.moviesite.com" might rank higher than "moviesite.com/actors" when keyword "actor" is concerned. But, for the keyword, "movie actor" the subfolder may rank higher (only speculations – not based on data). That might be the reason whey some have found subdomains to have less weight on the domain than directories. So, it is the choice of webmaster in determining which format they would prefer.
Some of the benefits I see of using subdomains over folder are:
- Separate detailed statistics for each subdomain – good to know exactly how it is or isn’t working for me.
- subdomains makes it easier to see what’s happening with visits and referrals (blog, affiliates etc.)
- some say sub-domain carry more weight with regard to back links.
- subdomains allow you to put your keywords at the front of the domain – adding more weight and prominence.
Taking Google as an example:
Subdomains – maps.google.com, news.google.com, groups.google.com, mail.google.com etc.
Subfolders – google.com/adwords, google.com/analytics, google.com/webmasters, google.com/adsense etc.
Separate sites (mostly, third-party purchases) – youtube.com, blogger.com, doubleclick.net
The best option is to build your site for your customer, not for the search engines. If your visitors prefer sub-domain, or if you think you can add value to your site by adding a subdomin, please do that.
My personal suggestions:
- if your content is closely related, use subfolder. In a juice theme site, "apples and oranges" can live together.
- If you are building sites for carrots and automobiles use sub-domains or buy a different domains.