Social bookmarking allows you to save links of your favorite sites which can be available for public viewing (this is how others can find your sites). When you add your site to a social bookmarking service, you create a backlink. When you submit an article to a directory, you also create a backlink to your site via the author bio. The difference is the effort you put in to create the backlink. Article marketing and social bookmarking are great free ways to generate targeted traffic to your website, so I would recommend doing both.