When looking at the characters of Medea and Jason, you can make many comparisons, which can turn into some very interesting essays. Here are some suggestions.
The first thing that you can do is compare their genders. This is a very fruitful comparison, because Medea is one of the strongest and most frightful women in all of Greek literature. She dominates even the great hero Jason.
The second thing that you can compare and contrast is in the area of natural and supernatural. Jason can be a symbol of what is natural and Medea can be an example of what is unnatural. She know about spells, magic and the like. It is her hidden craft that makes her so different.
If you probe these topics, then you will be able to come up with some ideas.