I think both did well in different ways as Dumbledore, but I give the edge to Harris. This is what I've said before:
Both Harris and Gambon were good for different reasons. Harris, I feel, does well as the wise, old mentor type, and I picture Dumbledore in the books being kind of like him. Gambon did well as the more energetic, almost "ageless" kind of Dumbledore, and, to paraphrase what somebody else said, was someone you could picture as the only wizard Voldemort would fear. I feel it would have been hard for Harris to pull off some of what the "later" Dumbledore had to do, and I think Gambon really did it well. Harris is my favorite Dumbledore, but I do like a lot of what Gambon brought to the role, as well.
