From what I know, articles are what we would think of as a reddit post. Naming conventions, and all. They are the things you read in magazines. Posts seem to go automatically under the microblogging section. So they're very much like a collection of tweets/blog posts aimed at a specific magazine.
Therefore if it's something important you want people to see, like a news article, something to fuel discussion, etc., I feel like doing it as an article would definitely be better. Doing it as a post is basically tweeting about it. Someone will probably see it, but most may miss it