Atticus is basically everything Bob Ewell isn't. He is a responsible, hard working, dedicated and thoughtful man. He is as dedicated to his children as he is to trying to do what is right but he also manages to consider the needs and situations of others as he strives to follow his conscience. What I find to be particularly important about him is his willingness to allow his children to learn on their own, though he knows better, he doesn't always just tell them which way is right or wrong.
Bob Ewell on the other hand, abuses his children, can't hold a job, and is basically a rather wretched human being. Apparently their family has wretched history and we don't know his whole story, so following Atticus' example we ought not to try and judge the man, but his actions are absolutely despicable, shown particularly when he attacks Scout and Jem.
Bob Ewell is the true antithesis of Atticus Finch. Atticus is considered one of the finest citizens of Maycomb--honest, trustworthy and an elected member to the Alabama legislature. Bob is a liar, a drunk and deliberately unemployed. Although both of their wives are dead (or did Bob's leave him?), Atticus takes care of his children, educates them and spends as much time as possible with them. Bob allows his children to live in filth, skip school, and apparently has little contact with them, leaving that chore to Mayella. Atticus is a friend of most of the people in Maycomb--black and white--and helps them when they are in need. Bob hates blacks, harrasses people, and probably has more enemies than friends. Atticus has a nice home in a residential area; Bob lives in a shack near the dump.
Bob on page 182 (I think) is described as being a 'little red rooster' which is the opposite on a Mockingbird.