Who is more ambitious at the opening point of the play, Macbeth or Lady Macbeth?

Macbeth never talks about his wife's ambitions, but she talks about his. Her soliloquy in Act 1, Scene 5 provides a good means to evaluate the relative strengths of their ambitions.

Glamis thou art, and Cawdor, and shalt beWhat thou art promised. Yet do I fear thy nature;It is too full o’ the milk of human kindnessTo catch the nearest way. Thou wouldst be great;Art not without ambition, but withoutThe...

Macbeth never talks about his wife's ambitions, but she talks about his. Her soliloquy in Act 1, Scene 5 provides a good means to evaluate the relative strengths of their ambitions.

Glamis thou art, and Cawdor, and shalt be
What thou art promised. Yet do I fear thy nature;
It is too full o’ the milk of human kindness
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it.
.......................................................................
Hie thee hither,
That I may pour my spirits in thine ear,
And chastise with the valor of my tongue
All that impedes thee from the golden round,
Which fate and metaphysical aid doth seem
To have thee crown'd withal.

Lady Macbeth obviously believes that her own ambition is stronger than that of her husband and that he will not act to become king unless she uses her persuasive powers. She is probably correct. We see throughout the beginning scenes that Macbeth has all sorts of misgivings about murdering Duncan. This should indicate that he is, as his wife says, not sufficiently ambitious to overcome his doubts and scruples. She is obsessed with the vision of becoming queen, but she cannot become queen until her husband becomes king. She overrides all his objections to going through with the plot to kill Duncan. She is so ambitious that she is blinded by her ambition. Macbeth is not blinded because he is not as strongly motivated. He sees all the possibilities of something going wrong and tries to explain them to her--but she won't listen. She believes or pretends to believe that all her husband's arguments against committing the murder are prompted by fear, which she stingingly calls cowardice.  It seems likely that Macbeth would not have killed King Duncan if his wife had not taken the lead, and this in itself seems to prove that she is more ambitious than her husband.