I have had XM for over a year and love it. One of these two systems will come to the top in the end and I"m betting it will be XM. You can do the billing by the year on a cc and I think you get a discount.
GM is now offering it as an option in many of their cars..
Performance has been very good with only a few minor signal drops in heavy trees or downtown areas with very tall buildings.
If your a NASCAR fan, they have MRN which carries all the races.