Depending on the title of an article, I can sometimes guess the opinion of the author and so I'm more interested in the HN commentary. If the article is something I know very little about, I will read that first.
It varies depending on whether or not I know the source (Hacker News displays the domain from which the link comes) and the subject of the article. Sometimes I read the article first, if I know the source is reliable or if I know I am interested in the subject, and sometimes I read the comments first to figure out if other HN participants, more knowledgeable than I am about the subject, think the article's treatment of the subject is worthwhile or not.
