I don't think you could find two characters that are actually more different. I think, however, the biggest difference that Shakespeare creates between these two characters is the way that Lady Macduff is explicity presented as a mother figure in the one scene in which she appears in Act IV scene 2. She is shown to engage in some pleasing banter with her eldest son, which is obviously designed to make the audience feel sympathy for her and her situation. In addition, when she is given news of the approaching murderers, note how she responds:
Whither should I fly?
I have done no harm. But I remember now
I am in this earthly world, where, to do harm
Is often laudable; to do good, sometime
Accounted dangerous folly: why then, alas!
Do I put up that womanly defence,
To say, I have done no harm?
Note the innocence expressed in her character through her response. She is deliberately presented as a good, perhaps somewhat naive, mother figure, who loves her children.
Contrast this impression with Lady Macbeth, who talks happily about smashing the head of her babe and can not be described as naive at all. The power of Lady Macbeth lies in the way that she is able to manipulate both her husband and others through a mixture of mockery and pleas. Whereas Lady Macduff is an honest and good character, Lady Macbeth fears that her husband is "too full o'th'milk of human kindness" and famously, in Act I scene 5, asks the spirits to make her into even more of an evil character than she already is:
Come, you Spirits
That tend on mortal thoughts, unsex me here,
And fill me, from the crown to the toe, top-full
Of direst cruelty!
It is interesting that Lady Macduff is a character that we associate with feminine motherhood, whereas Lady Macbeth, having no children and being asked to be "unsexed" here, stands in complete contrast to Lady Macduff.
Lady Macbeth and Lady Macduff are similar in that they expect their husbands be protect and rule their families. When Macbeth is considering whether or not to kill Duncan, Lady Macbeth tells him that he would be more the man if he were to go after his ambitions. She expects that he would do whatever is necessary to advance their family's position. Similarly, in Act 4 when Lady Macduff learns that Macduff has fled the country, she is angry with him for not staying behind to protect his family. She thinks that he has made himself look like a traitor, and she feels that he is now worthless in his role as husband and father. Despite these similarities, the two women are quite different in that Lady Macduff appears to have a good nature whereas Lady Macbeth exhibits a more evil, ambitious nature when plotting to kill Duncan.