If you already have the 960x640 (or better yet, 1135x640) articles, you
might as well use them, depending on your design and how much extra
production work you're willing to do. There is less letterboxing, and
the text is likely large enough to view. In my experience,
text-intensive iPad articles work on the iPhone 5/6 only if the body
text is at least 22 points.