[standard newbie disclaimer]
This may be a stupid question but I am a newbie to this arena so please go easy on me

[/standard newbie disclaimer]
Lets say I have a site that acts in part as a resource for a specific niche (and is ranked in the neighborhood of lets say 5) but I want to add another semi related niche to this site without starting a whole new domain that is seperate from it but not necessarily completely irrelevant to the content of the main site. would it be in my best interests to do either the following or something else?
create a subdomain for the new new semi related niche like:
keyword.mydoman.com/keyword/keywordphrase.html
or should i just do the following:
mydomain.com/keyword/keywordphrase.html
Sorry for the lack of information here, my thinking is still inthe conceptual stage and specifics are hard to come by
thanks for any insight you may be able to give on this.