First, Lady Macbeth shows herself to be sneaky and manipulative in her use of language. When she first learns of Macbeth's conversation with the Weird Sisters, she resolves to do and be whatever is necessary to bring about their promise. Therefore, her first words to her husband are flattering: "Great Glamis! Worthy Cawdor!" In contrast, Lady MacDuff, in the little stage time she is given, seems to lack guile. She honestly portrays her feelings, bluntly telling her son that his father is a traitor.
Additionally, Lady MacDuff differs from Lady Macbeth in that she is depicted with her son, and much of her time on stage is spent interacting with her son. The audience is therefore allowed to see her as a motherly person. In contrast, Lady Macbeth is never seen in a motherly context. Although she claims that she has nursed infants, there is no evidence that she has any children at all. Additionally, she claims that she would be willing to kill an infant while in the process of nursing if it would help her to accomplish her ends. She is not motherly at all.
However, Lady Macbeth does distinguish herself in her loyalty to her husband. She claims that all of the dark deeds that she commits throughout the play are for the sake of his advancement--she goes so far as to ruin her own sanity for his sake. Lady MacDuff is a little more complicated. She severely criticizes her husband for fleeing to England and leaving his family. On the other hand, she tells the murderers, when they arrive, that she hopes that he is "in no place so unsanctified where such as thou mayst find him." Clearly, she bears him no really ill will.