In the beginning of the play Macbeth is cautious and somewhat suspicious of the witches. He only starts to really think of the idea that they might be telling him the truth when they hail him as the thane of Cawdor, which shortly after, he receives word he has been named. When the witches tell him he will be king he is surprised but doesn't seriously consider killing Duncan until Lady Macbeth talks to him about it. This tendency to being easily convinced and manipulated shows that Macbeth is weak and has little ambition in the beginning of the play.
Lady Macbeth on the other hand is strong and very ambitious toward the beginning of the play. From the start she has control of Macbeth. When she read the letter about the prophecies and how Macbeth is coming home soon she immediately starts devising plans to kill Duncan. She isn't satisfied with Macbeth's newly acquired title of Thane of Cawdor. Only the crown will please her, and she is quite aware of what must be done to get the crown. This shows how she has a lot of ambition in the beginning. The fact that she can so easily convince Macbeth that he needs to kill Duncan proves that she is the stronger of the two in the early acts.
Even in the middle of the play Macbeth is still the weaker of the two characters. When the time comes to execute the plan and kill Duncan he tries to back out. However, Lady Macbeth once again easily convinces him into killing Duncan. He dreads the deed so much that he actually has hallucinations