The question is difficult to answer because it appears that it's the combined action of the witches and Lady Macbeth that initiates his killing spree. Oddly enough, however, one could also argue that neither she nor the witches are the real cause of the tragedy. Though Macbeth is urged on by the prophecies and the prodding of his wife, the seeds of his own destruction can be said to exist within him before the action of the play has even begun.
The reaction Banquo notices in Macbeth, seemingly one of fear, the moment the prophecy is uttered is, most people would agree, a sign that Macbeth has already considered killing Duncan in order to gain the throne. Therein lies the root of the tragedy and of the effect Macbeth's transformation has upon the reader and audience. He fears his own nature, the nature of a man who has the capacity to kill, and he knows that a monstrous guilt will eventually overcome him, even if he does not yet realize the specifics of how far he will go in his quest for power. The ultimate message of Shakespeare is that of individual responsibility and guilt. Regardless of the prodding of others, it's Macbeth's own fault that he does what he does. One could say that, on the surface, Lady Macbeth is the more immediate catalyst for at least the murder of Duncan, which then sets off the entire chain reaction of crime. But the basic question of which is the greater influence upon Macbeth is somehow irrelevant, in my view.
It's a paradox that Macbeth himself is responsible and, conversely, it is "fate," that is the cause of the tragedy, fate as a mystical force at the heart of the universe which cannot be resisted and which drives him on to kill against his own will. That these two possibilities can coexist is an existential dilemma. Man is accountable, but he is also helpless. I believe that our sense of this contradiction is what makes the character and play so great, so symbolic of the riddle of life. The witches and Lady Macbeth, in this interpretation, are mere adjuncts, props amid the real story of a man's tragic downfall.