There are several ways to compare and contrast Maggie and Tom Tulliver. They are the central characters of the novel, and they embody two very different outlooks on life.
Tom, the boy, is bound by tradition and is bent on achieving things that the world says are valuable. He wants to win the mill back for his father and the rest of his family, and give honor to his family name. Tom is much like his mother, who cares about appearances and wants to be seen as successful. But Tom is proud and intelligent, unlike his mother, and he clashes horns with contrary-minded sister.
Maggie's sensibilities are less of the material and social type than the emotional and aesthetic. Maggie, with her brains and headstrong nature, is easily swayed by emotions and her feelings for other people. She and Tom argue about many things, but Maggie's conduct with men is particularly what eventually causes a rift between the two.
Another tactic would be to analyze the processes of judgment in Maggie and Tom. When faced with dilemmas, what do each of them do? How is the process of decision-making carried out by each of them, and what values do they hold force them to make their decisions? What in their bringing up and education has prepared Maggie and Tom for their future adult decisions? This could include an analysis of their innate temperaments, but it might be even more interesting to show that how they were educated influenced the choices they made in life.
Another essay topic could be the way the two Tolliver children relate to their parents. Tom and Maggie have different relationships with their father, in particular, and it informs the way they act. Other essays might be about how Maggie can be seen as a pre-feminist heroine, with Tom representing the old patriarchal order, or Maggie as the progressive force of the changing industrial England, while Tom relates more to the rural past. Tom and Maggie, who at the beginning of the novel are close and loving siblings, become almost the antithesis of each other by the end of it. The events which cause this division each create a different response in Tom and Maggie, and an analysis of those responses would make a good essay.