This question could reasonably be answered either way. I'll give you some information on how each applies, and you can choose which one to explain in more detail for the essay.
1. tragedy of fate - The witches intervene and put Macbeth on a path toward destruction. Without their influence, Macbeth would never have felt compelled to go after the throne in the way he did.
2. tragedy of character - Macbeth ultimately chooses to make his own decisions. Yes, he is influenced by the witches, but at the end of the day, he is the one who leads to his own downfall by becoming power-hungry.
