Sub-domains, Separate Domains, or Sub-directories?

[miningreference]

We have a few websites that all deal with the Australian Mining Industry. Here are the sites:

Mining Reference
The Mining Jobsite
The Mining Blog
The Mining Showcase

What I am wanting to know is... Is it better to keep the sites as separate domains, or should I just make them subdomains (ie miningjobs.miningreference.com etc), or should I just make them sub-directories under the main domain (ie www.miningreference.com/miningjobs/ etc)?

In particular, I am looking at this from a Search Engine Ranking point of view.

Thanks,


Calvin

[incrediblehelp]

Never think in the terms of what's best for the search engines. Think of what is best for yourself to manage and what is best for your end visitors. If it is easier for you to do the previous on one website then do it. One thing to definitely consider is the history that these separate domains may have earned. The older the website/pages are the better they seem to do in the search engines. Also if these websites are doing well and getting traffic, why fix something that isn't broke?