I know sales aren't everything, but I thought it would be interesting to at least post this:
With all receipts available accounted for, Ayumi has sold a total of approximately 63,472,874 records globally, and Utada has sold a total of approximately 63,146,481. This means that the gap that separates their reported sales is less than 330,000.
Given that I stan for both, I don't know how I feel about this.