I like to stick with the standard meaning of the words used: An article
would apply to, well, articles. I would define blog posts, documents, and news articles as articles
. Sections on the other hand, would refer to layout/ux items: sidebar, header, footer would be sections. However this is all my own personal interpretation -- as you pointed out, the specification for these elements are not well defined.
Supporting this, the w3c defines an article
element as a section of content that can independently stand on its own. A blog post could stand on it's own as a valuable and consumable item of content. However, a header would not.
Here is an interesting article about one mans madness in trying to differenciate between the two new elements. The basic point of the article, that I also feel is correct, is to try and use what ever element you feel best actually represents what it contains.
What’s more problematic is that article and section are so very similar. All that separates them is the word “self-contained”. Deciding which element to use would be easy if there were some hard and fast rules. Instead, it’s a matter of interpretation. You can have multiple articles within a section, you can have multiple sections within and article, you can nest sections within sections and articles within sections. It’s up to you to decide which element is the most semantically appropriate in any given situation.
Here is a very good answer to the same question here on SO