Well, if you need help in identifying differences between the two characters, clearly a major difference between Calixta and Desiree is the way that Desiree is faithful to her husband and Calixta gives in to her desire and has an affair. Although Calixta is presented as a loving wife and mother through her concern for her husband and child, this concern masks the way that she has yielded to temptation with Alcee. Our overwhelming impression of Calixta is that of a sensuous creature driven by passion:
They did not heed the crashing torrents, and the roar of the elements made her laugh as she lay in his arms. She was a revelation in that dim, mysterious chamber; as white as the couch she lay upon. Her firm, elastic flesh that was knowing for the first time its brithright, was like a creamly lily that the sun invites to contribute its breath and perfume to the undying life of the world.
Note the explicitly sexual images that are used to describe Calixta. She is definitely depicted as a creature of passions and of desires.
Desiree, on the other hand, is depicted as a modest and loving wife and mother, whose only desire is to please her husband. Note how she is described:
What Desiree said was true. Marriage, and later the brith of his son, had softened Armand Aubigny's imperious and exacting nature greatly. This was what made the gentle Desiree so happy, for she loved him desperately. When he frowned she trembled, but loved him. When he smiled, she asked not greater blessing of God.
Thus Desiree is presented as a woman whose life is built around her husband and pleasing him. This is very different from the blatantly sensual way in which Calixta is presented. Thus, when we compare these two protagonists, it seems that Chopin is deliberately presenting one as becoming more sexually in touch with herself. Desiree, by contrast, is presented as the model wife, which of course heightens our sympathy for her in the way that she is treated by Armand and her eventual fate.