When writing a comparison and contrast essay, it is a good idea to begin by brainstorming. You can draw a Venn diagram of the two superheroes to begin. Look for things to describe each superhero, and then decide if that applies to the other one as well. For example, if you were writing about Superman and Batman, you might put that they both wear costumes, but Superman is an alien.
In addition to physical traits, you should also consider personality. Superman spent a lot of his time trying to blend in as Clark Kent. He used his powers to help people who needed it but kept his real identity a secret.
Once you have brainstormed, you need a coherent thesis. You can argue that one of the superheroes is better, that Batman is an antihero, or any other argument. Then you will use your brainstorm to create an outline supporting the thesis. Remember that the goal is to convince your reader of what you are trying to say.
My best advice is to choose characters you know well. This way you will be passionate about the subject, and you will not need to do a lot of extra research that will get you confused.