I would choose one overall aspect to compare the two systems of government. Essays need a strong thesis, followed by a well-developed argument. Choose two systems of government and make an overarching statement about them, and then discuss about three key areas of distinction.
Here's how I would set up this answer:
First, I would look for a number of ways in which the two systems of government are different. The body of my paper would consist of a discussion of these differences. I would do this before I wrote an introduction or conclusion.
Second, I would look at the differences I had just laid out. Then I would decide which of the two I thought was better based on those differences.
Third, I would go back and write my introduction and conclusion. Those would talk about which form of government I thought was better and (briefly) why I thought it was better.