These two methodologies are not so different, but they have a crucial variance: blogger outreach makes the mention of the link seem hugely more natural. In guest posting services, the link is usually at the author´s bio, reducing the number of readers drastically (many of us don´t read bios?).
With the blogger outreach approach, on the other hand, we contact active and highly relevant bloggers for collaboration on a piece of content that includes the editorial link within the body and flow of the article, naturally.
The content we offer gets published with the blog´s owner signature on it. Readers already trust him or her to make the bond and Google takes this as a sign of authority allowing your websites to rank better than ever.