subdomain vs non-subdomain

I am the owner forums.intpcentral.com but it was originally at domain.com/forums/

I get 102k results with using site:intpcentral.com and 25k results with site:forums.domain.com

I haven't yet purchased vBSEO but I was trying to figure out what approach I should be taking to try to consolidate those 2 URL's. If I was to make a guess, I'd need to have a 301 redirect to whatever the preferred URL would be. Since forums.domain.com doesn't have as much indexed, I'd suppose it will be wiser to use the foldered URL.

It doesn't matter. It's all the same, I think.

Why doesn't it matter? From what I've vaguely read it is important to not have duplicate content (such as it would be the case if someone used multiple domains for the same content), so how is this any different?

subdomain.yoursite.com and yoursitecom/subdomain both are 2 different URIs for search engines, you will face duplicate content issues due to this.