Both civilizations were, it would seem, rigidly hierarchical, and religion played an important role in both. An interesting topic might be to compare and contrast exactly how and why the two civilizations responded as they did to the threats they faced.
Researching the general topic of the Aztecs and Mayans should be relatively easy, because both civilizations are well known and were highly important. One factor to keep in mind is that all civilizations evolve over time, so you want to make sure that you are comparing and contrasting the two cultures at comparable periods of their development.
It's easy to forget that eNotes itself has plenty of good material available on practically any topic. I thought I would do a little digging for you and list the links that looked most helpful. Always be sure to look for the bibliographical information cited in any source.