I feel like Madonna will end up outselling MJ in the end, if her next few records constitute a resurgence similar to ROL, Music, and Confessions; I also believe that they will remain the two most successful solo pop artists of all time due to their volume of albums sales.
It will be interesting to see if artists like Rihanna do ever actually pass them in records sold due to the advent of the digital age, but it looks like album sales will never recover enough for anyone to surpass them overall.