SEO: Subfolders or Subdomains
I recently moved my blogs from subdomains and multiple domain to one domain with subfolders, here are articles on the subject from Chewy's Blog and Search Engine Journal
... 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