I think Sirius will win the competition for listeners due to one thing, their superior content. They paid the big bucks and now they have a much better lineup of channels. Howard Stern (both channels are really good, he is adding other people to them such as Danny Bonaduce), NFL, Nascar (coming shortly), Martha Stewart, and a ton of very good music channels.
XM has sort of dropped the ball on content. Forget Oprah, she is only going to call in for 30 minutes a week. It is a joke how little effort she is putting in to the channel after being paid so much money.
I do agree the data content on XM is great and worth a few hundred thousand subscribers for things like real time weather and traffic on your Garmin 396.