My students read chp. 11 of "The Lightning Thief" - about Auntie Em aka Medusa and then read a kid-friendly retelling of the Greek myth Perseus and Medusa. We used these sheets to compare and contrast -CC standard RL 6.9. This activity was also great for working on annotation, tone and mood, setting, plot, and word connotation. Just depends on what you focus on ;)
There are graphic organizers for the myth, The Lightning Thief, a key differences sort, and an ERQ for when you are finished with this activity as an assessment.