What would roll down a hill faster: frozen diet soda or not frozen?

-->Not Frozen.

Look at the different forces acting in each case.

When liquid, the can accellerates down the hill. The container will be turning, but the liquid inside won't rotate much. Any rotation of the liquid inside is caused by the (minimal) friction between the inside of the can and the soda.

When frozen, the soda is fixed relative to the can, and in order to roll down the hill some force is wasted accellerating the frozen liquid in the can rotationally.

If you neglect friction, the answer is simpler. When frozen, energy must be spent to rotate the can, as well as it's contents. When liquid, the can has the same weight, but can essentially slide down the hill, almost as if the hill itself was lubricated.

Go with the frozen, there is less friction. The fluid in motion would cause you to lose momentum. But, it does depend on whether a force is applied other than gravity.