After the first encounter with the Weird Sisters, Banquo warns Macbeth:
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence
Essentially, the vision of the supernatural we get from the three Weird Sisters is one of malevolence. They are fundamentally evil, and like Satan (whom they serve) tempt men to evil deeds.
Puck, on the other hand, is mischeivous, but essentially neutral. Although he is a classic trickster, and enjoys jokes, he acts without active malics, and in fact serves to unite the lovers and bring about a happy ending to the play. Thus we get a more morally neutral vision of the supernatural from Puck.