There seems to be varied and conflicting opinions on this based on what i have searched through on the web.

Does anyone here have SEO experience and thoughts on this ?

If rationalizing website content, would you go for a sub-directory layout, or would you use sub-domains ?

I would be grateful for any comment and your reasoning as to which choice you would prefer.

As the web (v2.0) has moved toward CMS driven by databases, the end-user and search engines doesn't have a clue about sub-directories vs sub-domains.

If you look as ThaiVisa as an example, heavily reliant on CMS, searching for the term 'ThaiVisa' will display the main www sub-domain, and list some of the more popular Forums, but except for the www. and classifieds. subdomains, the other subdomains used go unknown. This may be by design, but finding out how to advertise on ThaiVisa gets stuffed in the closet. I actually have to add "advertising" to the search request to get it to reveal the marketing. subdomain.

The answer is, depends.

If you're creating a semi-independant site under the sub-domain (e.g. targeting a specific geographical region/market, or a different vertical/department), and both primary domain, and sub-domain have plenty of content (say, >50 pages each), then yes, sub-domains can be a good strategy.

If the site is small, partitioning off pages to a sub-domain will likely be detrimental - small sites will never perform well on Google's "subject authority" type signals.

There is absolutely no difference in SERP if the keyword you're targeting is in a sub-domain or the first segment of the main domains path. Up until about 2 years ago, sub-domains would get you better SERP than a path, but that's all changed.

In short, for a big website, use either - but even then, think hard before you go the sub-domain route, and only do it if it makes sense *for your audience* (e.g. www.jblpro.com vs. asia.jblpro.com). For a small website, use a path over a sub-domain so your primary site has more content.

Bottom Line: You should never use sub-domains as an SEO tool.