The difference is the restatement of the antecedent in the second example you give. You used "article" twice. Since "which" is a pronoun, it can stand in for t

The difference is the restatement of the antecedent in the second example you give. You used "article" twice. Since "which" is a pronoun, it can stand in for the actual noun, as it does in your first example. But sometimes the noun is restated along with the pronoun. Here's another interesting example: "Which article didn't you understand?" In question form, the "ante"cedent of the pronoun would actually come long afterward, even from a different speaker, as the answer to the question. But to add clarity to the scope of the question, a noun summarizing the answer type ("article") accompanies the pronoun.