A sub-domain will be viewed by the search engines as a different URL altogether.
If your domain has good ranking, I'd opt for a sub-folder over a subdomain.
I don't buy part of this article..
"Google has changed how it views subdomains. Google now views the subdomain as part of the main domain and not as a separate entity, which means the SEO implications of where you put your content are not as crucial as the quality of the content itself.
I've also read a few times that it depends on the content and interlinking between the main domain and the sub domain. Meaning, if you have a blog in a sub-domain and link heavily between it and the main domain, then Google will treat it as part of the main domain.
Google's page rank algorithm is what you should be concerned about. Basically your rank goes up when other highly ranked sites publish your link. That means you need good content on your site that is worth linking to, and then you need to try to get published.