[Duncan] not resembled/My father as he slept, I had done't. (II,ii,12-13)

When Macbeth returns, telling her that the servants cried out "Murder!" and "Amen" stuck in his throat as they prayed, Lady Macbeth replies,

Consider it not so deeply..../ These deed must not be thought/After these ways; so, it will make us mad. (II,ii,29-33)

Still, Macbeth is uneasy, explaining that he heard a voice cry, "Macbeth shall sleep no more" (II,ii,42).  He then tells his wife that he cannot look upon the daggers.  Angrily she retorts,

Infirm of purpose!/Give me the daggers.  The sleeping and the dead/Are but as pictures. 'Tis the eye of childhood/That fears a painted devil....I shame /To wear a heart so white..../A little water clears us of this deed. (II,ii,52-66)

This last line is, of course, ironic.  For, Lady Macbeth is unable to really not think of the deed as she has instructed Macbeth, nor can she use water to clear them of their murderous deeds.  In Act V, obsessed with guilt that she can no longer push out of her thoughts, she seeks to wash out blood that she imagines is on the stairs of the castle in Dunsinane.  The character who has accused another of weakness has herself gone mad because of her guilty feelings.  Despite her earlier conviction that she could push them away by simply not contemplating them, she is not strong enough to do so; her conscience gets the better of her, driving her insane and, finally, to suicide.

Who, then, is stronger?  If one defines strength as having the will to pursue one's goal despite its evil, Macbeth is, then, stronger because he continues on his murderous path toward power after Lady Macbeth succumbs.  If, on the other hand, one defines strength as moral integrity, Macbeth certainly lacks this much more than his wife.

In my opinion, Lady Macbeth has the stronger character, but it is more brittle.  You know how sometimes people talk about flexible things being stronger because they bend but don't shatter?  That's how I see these two.

In a sense, Lady Macbeth is stronger.  She is the one who pushes her husband to kill Duncan and do other things necessary to become king and keep his power.  She shows more strength then because she is more willing than he to do what is necessary.

But in another sense, her character is more breakable.  She is the one who goes mad and kills herself while Macbeth does not.

Even though Lady Macbeth prods her husband on, she has no actual power.  Her only power is in pushing him.  This is why she pushes him.  She wants to have the power of being queen.  Malcolm calls her the “fiend-like queen,” but in reality Lady Macbeth has little to do with Macbeth’s kingdom.  She helped with the initial murder of Duncan, and encouraged him to kill Duncan, but after that she was not involved in the other murders.

Lady Macbeth is inspired when she finds out about the witches’ prophecies that Macbeth is going to be king.   She knows that her husband wants this.  However, she worries that her husband is too wishy-washy to act on his 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 … (Act 1, Scene 5)

Later, Lady Macbeth does plan everything for her husband and chide him when he doesn’t follow the plan to the letter.  She seems to know what she is doing in planning murders.  However, she doesn’t carry out the murder herself.  She needs him.

LADY MACBETH

Alack, I am afraid they have awaked,
And 'tis not done. The attempt and not the deed
Confounds us. Hark! I laid their daggers ready;
He could not miss 'em. Had he not resembled
My father as he slept, I had done't. (Act 2, Scene 2) 

Relying on her husband must have been hard for such a strong woman.  She turns out to be not as bloodthirsty as she seemed.  Lady Macbeth starts to crack.  Macbeth, it could be argued, was cracking long before he killed Duncan.  He was already envisioning floating daggers.  By the time Macbeth has Banquo killed and sees his ghost at the banquet, Lady Macbeth is making excuses for him.  It won’t be long before she loses her tenuous grasp on reality and is trying to wash her hands of blood long after it is no longer there.

Both Lady Macbeth and her husband exemplify qualities of human frailty, rather than simply those confined within their respective genders. Lady Macbeth is a woman ahead of her time in her clear focus on the ultimate promotion for her husband (remembering that we are still in an age which believes in the Divine Right of Kings). She is Machiavellian in her desire for her husband, and through him herself, to be all powerful. However, the judgement of Shakespeare is that she steps beyond strength and determination into evil and immorality, and this is why she pays with her life. Similarly, Macbeth is a brave and courageous warrior, well deserved of the honours which are bestowed on him legitimately, but brought in to check when he tries to take what he is not ordained to have: the throne of Scotland.

The characterization of Lady Macbeth in the beginning of the play reveals the fundamental sexism in Shakespeare's time, one that surpasses the accepted roles of men and women in his society. Women were seen as beings whose fundamental physical nature was different from that of men in that they were programmed by nature itself to be physically weak and emotionally nurturing. Men, in contrast, were accepted as being programmed by nature itself to be strong and emotionally bent toward physical conflict and cruelty. In contemplating Duncan's murder, Lady Macbeth first seeks supernatural intervention to change her basic feminine nature so that she will be capable of the cold cruelty necessary to kill the king. She also worries that her husband lacks the natural cruelty of the male to carry it out. So, in the context of Shakespeare's audience, Lady Macbeth is "more of a man" than her husband in the beginning.

Once the king is murdered, however, he quickly makes up the difference. Macbeth goes on to commit a series of vile murders without his wife's prior knowledge, and she reverts to her own basic feminine nature. In the play's conclusion, she is weak and emotionally undone by the horror of the crimes.

I think that the previous posts do a great job of exploring the characteristics of both characters.  I would caution that the phrasing of the question might seek to do what Shakespeare himself is seeking to avoid.  One of the most compelling elements of Shakespearean work is the idea of how the human predicament is revealed.  Whether by design or unintentional, he seeks to create a realm where individuals are not bound by gender as much as their own frailties.  The mistakes and errors they possess are, to a point, universal, ones that can be held by anyone in the "perfect storm" of circumstance.  Both husband and wife possess flaws that seem to work off of one another.

Whenever a reader critiques a literary work, he/she must always consider the time period in which this work has been written.  For instance, in Shakespearean times, there was no woman suffrage or liberation.  Often women's roles in drama are to be supportive as are Portia in "The Merchant of Venice" and Portia of "Julius Caesar" along with many others.

Now, in "Macbeth," Lady Macbeth goes beyond being supportive to taking the commanding role, one usually reserved for the male.  Thus, this perversion of roles carries on the motif of the play:  "Fair is foul."  So, while Macbeth waivers--"If it were done when 'tis done, then 'twere well/It were done quickly"( I,vii,1-2)-- as a stereotypical woman would, his wife is decisive and assertive, traditionally male characteristics.  She accuses of Macbeth of having lost his manly traits:

When you durst do it, then you were a amn;/And to be more than what you were, you would/be so much more the man.(I,i,47-51)

She accuses Macbeth of weakness and declares her "strength" and resolve by declaring that if she had decided to take action against Duncan as Macbeth has, she

would, while it was smiling in my face,/Have plucked my nipple from his boneless gumes,/And dashed the brains out, had I so sworn as you/Have done to this. (I,i,56-58)

Then, when Macbeth yet falters, asking, "If we should fail?" Lady Macbeth counters,

We fail?/But screw your courage to the sticking-place/And we'll not fail...(I,i,59-60)

Certainly, Lady Macbeth is crueler and colder than her husband; and, taking murderous action, she assumes the dominant role, and, as such, appears more manly, especially when compared to her husband whose reputation as a warrior who is never hesitant in battle.  But, is she truly more manly than Macbeth?  In essence, probably not.  She simply embraces evil sooner than he.

I would think Macbeth to be more of a man due to the fact that he actually thinks through the consequences of things prior to them happening.  He feels the guilt and understands that there is an issue, however Lady Macbeth just thinks of the power and greed that lay ahead. Macbeth is more of a man for this because he uses logic prior to taking action.

Macbeth is also able to follow through with many of his promises while Lady Macbeth really just forces her husband into it.   Why doesn't she just do it herself? Because she feels that King Duncan looks too much like her father.   She sets things up but cannot do them herself and cannot keep herself together after the actions are complete.  She appears to be feeling extremely guilty and almost insane when she sleepwalks and eventually kills herself.  Macbeth on the other hand is able to handle the consequences of his situation after it is complete.