To my knowledge, what matters Google pagerank-wise is a page, not a web site. However, if you are going to provide individual homepages for the users of your w

To my knowledge, what matters Google pagerank-wise is a page, not a web site. However, if you are going to provide individual homepages for the users of your web site and you want their URLs be mycompanyurl.com/username, like with del.icio.us, then maybe the domain option would be a better choice, and probably not only for your blog but for the rest of your corporate info as well.