I think people should stop with the Madonna comparisons. Yes she's sold more than Kylie so good for her.
Kylie's done well in her own right. Beside's, Kylie is 10 years younger than Madonna. Why not compare her to JLo/Mariah instead who are more of a similar age.
Women are compared more than men in the pop world it seems and their worth is judged in terms of their age in a way that it's not with male artists. Which explains why Madonna's peak on the singles chart may have ended before Kylie's, because she's ten years older.
The only woman who defied that rule was Cher, but she never had a smash like Believe again. That was a fluke.