When creating a rubric for a writing assignment, you want to be clear about how students should demonstrate their knowledge (content) through these means (writing). You will also want to properly balance what you are emphasizing whether that is the knowledge of the issue (content) or the means by which that knowledge is communicated (writing).
For instance, if your purpose is to assess whether or a not a student has researched a complex debate in your field, then you may place more emphasis (and grade weight) on structure and content. The structure will tell you if a student has indeed explained all sides of the debate and the content will tell you if they did so proficiency. On the other hand, if your purpose is to assess whether a student has explicated that debate, then you may emphasize (and weight) the delivery more than the structure.
We have provided two example rubrics from our English faculty that can use as a model or adapt for your own assignment.