I'm thinking of creating a family of websites which use the same engine, but target different topics.
Given the choice is it better to have somekeywords.com, somekeywords2.com or myotherwebsite/some-keywords, myotherwebsite/some-keywords2.
For example: stackexchange.com/stackoverflow stackexchange.com/askubuntu instead of stackoverflow.com
The best is the shortest and the easiest address to be remember by your users.
By my side, my favorite practice is usage of sub-domains:
BUT, a lot of users want absolutely put WWW before when I stated orally (for example, by phone), so, I always make an other sub-domains like this with redirection just for this purpose: