What are the differences between Mayella and Mr.Ewell's testimonies that can be used to defend Tom Robinson's innocence?To Kill a Mockingbird by...

Significantly, Mayella's testimony differs from her father's.  For one thing, in Chapter 18, she testifies that Tom Robinson choked her as she faced him, then he "took advantage of her."  She says nothing about having been struck in the right eye (or the left, for that matter) as her father, Tom Ewell, has done previously in Chapter 17.  And, when Atticus questions her, "Do you remember his beating you about the face?"  Mayella hesitates.  When Atticus reminds her that she seems certain about the choking, and asks her again about being hit, she replies,

'No, I don't recollect if he hit me.  I mean yes I do, he hit me.'

'Was your last sentence your answer?' [Atticus]

'Huh? yes, he hit--I just don't remember, I just don't remember....it all happened so quick.'

Unlike Mayella, Tom Ewell has been absolutely affirmative about Mayella's eye having been blackened.  He also has said nothing about Mayella's neck being bruised, red, etc. which would have indicated choking, whereas she has repeatedly testified to this action.

Obviously, there is a question of credibility with the witnesses.  In addition, Mayella is vague about when her father arrived on the scene,

I don't remember too good, but next thing I knew Papa was in the room a'standin' over me hollerin' who done it, who done it?"  

whereas Mr. Ewell has stated that he looked through the window and saw Tom Robinson on top of his daughter. Contrary to this is Mayella's citing her father's having asked "Who done it, who done it?"

These discrepancies between the father's and the daughter's testimonies, along with the fact that Mayella possesses a right black eye which indicates that a left-handed person must have hit her (her father is left-handed and Tom's left arm is useless) are what Atticus Finch believes will prove the innocence of his defendant. 

Also, with one of the main themes of Lee's novel as that of Prejudice, the description of the hearings of Tom Robinson's trial, in conjunction with the verdict of the jury, clearly demonstrates such prejudice.  Indeed, the testimony of Bob Ewell and of Mayella are pivotal to the development of the theme of Prejudice, a development that becomes apparent after a close reading.