Aunt Alexandra and Calpurnia are both strong female figures in To Kill a Mockingbird. So, they both play a mother role, more for Scout than for Jem. Aunt Alexandra is conservative and although she shows some compassion with her missionary circle, she lectures Scout on the prestige of her family's history and this goes hand in hand with preserving social (and racial) class distinctions. However, Aunt Alexandra does get upset upon hearing news of Tom's death. There is the opinion that Aunt Alexandra was more of a traditionalist than an outright elitist. That is to say that any residual racism she exhibits is a product of her loyalty to those social traditions.
Calpurnia is much more open-minded than Alexandra and makes a larger impression on Scout both as a mother figure and as a guide to the society and culture of Maycomb. Calpurnia is the bridge between the white and black worlds of Maycomb. When she takes the kids to her church, this is the first time Scout considers that Calpurnia has a life outside of the Finch household. This is a moment where Scout really begins to become aware of other people's lives rather than thinking only of the role they play in her (Scout's) own life.
“That Calpurnia led a modest double life never dawned on me. The idea that she had a separate existence outside our household was a novel one, to say nothing of her having command of two languages” (67).
Calpurnia scolds Scout for making fun of Walter Cunningham at the beginning of the novel. This is the first example of Calpurnia teaching Scout to put herself in other people's shoes. This is a lesson that Atticus instills in the children as well.
Calpurnia, like Atticus, is a voice of reason and represents the social separation or double-consciousness of living in a white and a black world. Aunt Alexandra represents a traditionalist of the south who is reluctant, but not totally unwilling, to give up social traditions even if they are in the spirit of an historical evolution towards equality.