One thing I'd add, I guess it depends what you mean by customers. I'm not really considering online, as that will vary by days of the week, time of day, etc. but people using the service to play, buy games, etc. reasonably regularly.
Steam sells a fair amount of SP titles as well as online, and I'd count PC gamers using it to buy SP games customers, even though they won't show up that often using online stats.
So in total I suspect WW there are more people using Steam to buy, trial or play games and that as a result of the strength of its DD side it probably sells more (although it doesn't have the additional Live subscription fee so that probably evens things out a little vs Live).
Try to be reasonable... its easier than you think...