An "article" is supposed to be "informative" whereas a "post" has traditionally been more "opinionated."
An article is generally longer. A post is often much more informal.
I was told the other day by an SEOer that Google doesn't make a distinction between articles and posts (or even something very short called "blurbs") anymore.
Why is it important for you to decide on the exact format, CoolBee? I believe one of Steve's posts (or "articles," as he also calls them) addressed this particular point, and I think his take-away was that he just writes to the best of his ability, with his goals in mind, which also involves his audience....