Characterize the relationship between Macbeth and Lady Macbeth. Whose ambition is the driving force of the play—Macbeth's or Lady Macbeth's or both?

Both of their ambitions are driving the play forward for sure.  Macbeth begins considering the throne moments after he hears the witches' prediction.  If Macbeth wasn't ambitious for more power and prestige, he would have disregarded the witches as raving lunatics.  

However, Macbeth's desire for the throne isn't actually enough to convince him to take any kind of action toward it.  For that, he needs Lady Macbeth's driving ambition.  Lady Macbeth recognizes that her...

Both of their ambitions are driving the play forward for sure.  Macbeth begins considering the throne moments after he hears the witches' prediction.  If Macbeth wasn't ambitious for more power and prestige, he would have disregarded the witches as raving lunatics.  

However, Macbeth's desire for the throne isn't actually enough to convince him to take any kind of action toward it.  For that, he needs Lady Macbeth's driving ambition.  Lady Macbeth recognizes that her husband is ambitious, and she knows that he won't do anything foul to achieve greatness.  She speaks exactly those thoughts in Act 1, Scene 5.

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: what thou wouldst highly,
That wouldst thou holily; wouldst not play false,
And yet wouldst wrongly win: thou'ldst have, great Glamis,
That which cries 'Thus thou must do, if thou have it;
And that which rather thou dost fear to do
Than wishest should be undone.'

Lady Macbeth correctly identifies that her husband wants greatness and power, but doesn't have the devious spirit to win at all costs.  That's why she wants Macbeth to hurry home.  She wants to begin convincing Macbeth that he must kill Duncan.  

Hie thee hither,
That I may pour my spirits in thine ear;
And chastise with the valour of my tongue
All that impedes thee from the golden round,

I do put a lot of the blame on Macbeth for killing Duncan.  After all, he is the person that actually did the murder.  But, without Lady Macbeth pushing him, I don't think that Macbeth ever would have gone through with it.  She most definitely wears the pants in that relationship.