Distribution medium - which affects how you reach or target your audience. On a blog, you may use the first person strong editorial voice and be informal, because the readers will have found your blog via a search or through your home page, they're expecting to be reading a person or company's blog, they're preconditioned to know what that is, so you may not need as much as a setup to your point (just an example) . An article, on the other hand will be written for a different distrubition medium which may require certain conventions like objectivity, formality, or with an expert tone (not saying a blog entry can't), but it will usually be targeted at people who have no idea who you are and have no connection to the page or author. The lines are blurry, but think audience and distrubtion medium. Where is the article going to show up - newsletter, article directory, nation publiction , this will effect it's tone, style, and voice. Also remember that blog entries and articles can (and frequently are) the same given the bloggers voice works for article sydication. For example, I co-author a search engine marketing blog, Stir Crazy and the individual entries are syndicated by WebProNews under "blog talk", but they have also taken certain blog entries that were in the correct voice and labeled them as "articles" under one of their other publication "searchnewz."
I feel that blogging can generate link baits much better than articles because of the ongoing interaction that a blogger has in the blogosphere or niche community of bloggers. Just my take on that..
i would use both in your marketing strategy. market article to directories and it'll get spead virally, something that most are still trying to get down. but it's got to have uniqueness to it though. blog and sociallly bookmark your blog posts so other can view your content and get some visitors.