... search engine view subdomain site as a different site so the work you have done on the sub domain does not directly affect the parent domain as they are viewed as a separate entity.

History, first I had my blog.baleinoid.fr and whaly.baleinoid.fr websites, I moved recently whaly.baleinoid.fr to wha.ly for the fun of the new domain now I moved again and I have http://baleinoid.com/journal/baleine and http://baleinoid.com/whaly

I checked the pro and cons and I choosed the subfolders path, because I don't have a lots of backlinks and it's easier to get a better search engine rank with one domain.

Subfolders Pro:

• to spread/share the rank of the domain

Subdomains Pro:

• if you have different hosting server for your blog / forum / website