Which is better: fasting on the Day of Arafah or fasting on the Day of Ashura, and what is the evidence for that?

Fasting the Day of Arafah is better than fasting the Day of Ashura, because it expiates sins for two years, whereas fasting the Day of Ashura expiates sins for one year. The proof for this is the saying of the Prophet, peace and blessings be upon him: "...Fasting the Day of Arafah, I anticipate that Allah will expiate the sins of the year preceding it and the year following it. And fasting the Day of Ashura, I anticipate that Allah will expiate the sins of the year preceding it." It has been said, regarding the wisdom behind this, that the Day of Arafah is attributed to the Prophet, peace and blessings be upon him, and the Day of Ashura is attributed to Moses, peace be upon him.