The main difference between the Cunninghams and the Ewells is one of ethics. The Cunninghams have personal pride and dignity, and they possess respect for people and the law; the Ewells have none of these qualities.
In Chapter 23, Atticus states that the Cunninghams are a family with a long history of possessing principles:
...the Cunninghams hadn't taken anything from or off anybody since they migrated to the New World..
Whenever Mr. Cunningham needs the legal services of Atticus, he repays his debt with produce from his farm. His work ethic extends to his children, as well. For instance, when Jem invites Walter to eat lunch at their house in Chapter 3, Walter explains that he is unable to pass to the next grade at school because he has to work in the fields at home. In Chapter 15 Scout accompanies Jem to the jailhouse only to find her father confronted by the threatening mob and she approaches Mr. Cunningham, asking him about this "entailments." She also reminds him of who she is and mentions that she brought Walter home for dinner (lunch) not long ago. These reminders of kindness by the Finches to both him and his son Walter touch the decency of Mr. Cunningham, who then tells Scout that he will say hello to Walter for her. "Let's get going, boys," Mr. Cunningham says, then he and the others go to their cars and drive away.
Unlike the Cunninghams, Bob Ewell and his family have no respect for those in authority. In Chapter 2, when the dirty Burris Ewell is questioned by Miss Caroline on the first day of school, he is insolent and disrespectful. He retorts,
"Ain't no snot-nosed slut of a school-teacher ever born e'n make me do nothin'!"
Burris only comes to school on the first day because he does not feel like going, not because he has to work for the family, as Walter Cunningham does. His father, Bob Ewell, does no work, either. Instead, he receives a welfare check from the government and he drinks and is dissipated. Not only does Ewell neglect the physical welfare of his children who are hungry and dirty, but he abuses his daughter, Mayella, sexually and physically, as well. Mayella is left alone to care for her little siblings. (Chapter 17)
In Chapter 17, also, Ewell has no qualms about charging Tom Robinson with rape when he knows full well what has happened. He perjures himself and makes his daughter Mayella perjure herself in the courtroom at the trial. After Atticus proves him a liar, Ewell spits in Atticus's face and tries to bring great harm to the Finch children.
The Ewells are unprincipled, vindictive and even murderous; however, the poor Cunninghams are respectful and decent.