An Informative Abstract is informational and direct. It is narrative in nature and provides the reader with the base contents of the article. In many cases it obviates the necessity for reading the article as the contents of the article are made clear in the abstract. (This is the kind of abstract that should be written for many class assignments.)
A Descriptive Abstract is to alert readers to the existence of an article and bring awareness of the article. It is a sort of "current awareness" notice, that brings attention to articles that may have been overlooked. It is not judgmental. The reader makes the choice about the importance of the article for themselves.