Agree with above. Directly on rack circulates the air around the bottom & produces a better crust.
That said, I personally use a stone because I find it easier to get the cooked pizza out of the oven. Do not pre-heat the stone (putting a frozen pizza on a hot stone = cracked stone). The stone absorbs moisture creating a crispier crust than a pan. I also use the stone for baking bread, rolls, and biscuits.