When I spoke with a Best Buy sales rep earlier in the year he explained the differences between XM and Sirius very well. Can't remember his words but he did mention no ads at all with Sirius. That was about it for the positive. XM on the other hand has been available to the masses for a while, and is supported by many brands. Sirius is a kenwood product...I believe.
I guess what I'm saying is go ask someone in the know.