Having been to both sites I would recommend Chichen Itza. Did you know that ignorant Americans have been known to refer to it as "Chicken Pizza"? There is the Sacred Cenote, the Temple of the Warriors, the Astronomy, tht Main Pyramid and much more.
Uxmal is noted for its friezes not its architecture. They had a light and sound show after dark that highlighted the friezes and had a commentary that was pure corn. "They came from the east, they came from the west>"
My choice would be Chichen Itza.
