Love this question.
The main difference between the two families is their expressions of moral values.
The Ewells will live off the welfare of the county (and have done so) for generations. They are lazy and take any and everything they can get for free. They do not contribute to society.
The Cunninghams have pride. They will not take something they can't pay back and have a demonstrated history of paying Atticus back in produce as opposed to dollars.
Atticus allowed a Cunningham to be on the jury because of what had happened the other night at the jail. Atticus believed that the Finches had once again earned the respect of the Cunninghams and that there was chance that one of them "took considerable wearing down" for the jury to be out that long. Atticus leads us to believe it was the Cunningham that he did not strike from jury selection.
Scout earned that respect when she treated Walter Cunningham Sr. like a human being that night.
I don't believe Atticus makes any personal comments about the Cunninghams or Ewells during Chapters 19-22. However, he talks with Jem and Alexandra about Ewell in Chapter 23. He explains to Jem that he
"destroyed his last shred of credibility at that trial, if he had any to begin with. The man had to have some kind of a comeback; his kind always does."
Atticus reassures Jem that Bob "got it all out of his system that morning" when he spit in the attorney's face.
Concerning the Cunninghams, Atticus only hints that one of them was on the jury. He did go on to say that
... the Cunninghams hadn't taken anything from or off anybody since they migrated to the New World... once you earned their respect, they were for you tooth and nail. Atticus said he had a feeling... that they left the jail that night with considerable respect for the Finches... it took a thunderbolt plus another Cunningham to make one of them change his mind. "If we'd had two of that crowd, we'd've had a hung jury."
Atticus says that "On a hunch, I didn't strike him" (the Cunningham "connection") from the jury selection.
Earlier in the novel, Atticus claims that "the Ewells had been the disgrace of Maycomb for three generations." As for Walter Cunningham Sr., Atticus knows he would eventually be paid for his legal services. "... before the year's out, I'll have been paid. You watch," he told Scout.