According to this site, the XM 20K has higher PPFD than the 15K, check near the bottom of the page, there is a comparison chart.
http://www.advancedaquarist.com/2005/6/aafeature2/view
I am also surprised that the name brand ballasts seem to get lower PPFD on average, than the "standard" magnetic M59 type ballast (not sure what brand "standard" is though!)