Comment on Website Navigation Structure SEO Guide by delta.

Design a Websites Navigation Structure That is another very interesting question for me:

Stallion WordPress blog installed in subdirectory or subdomain

I would like to install an additional blog to my Joomla site in a subdirectory like: | or |

What are the SEO implication thereof?
How will Stallion influence the overall performance?
Will I treat Stallion as a separate “SEO entity”?

And could you please add some advise on the endless discussion about the preferred domain name:

Which is better? | or |

Does it even matter anymore? My question here is: If you are asked to make a choice, why doesn’t Google treat both possibilities as equals or “synonymous”?

Thanks a lot!