I've researched both companies and it seems like Sirius is the better. They may have started subscriptions later than XM (though they did get their 3 satellites up beore XM's 2 satellites) but they have agreements with some of the major car companies like BMW, Mercedes, the entire ford lineup including mazda, jag, land rover etc and others to be factory installed. So far GM is the only MAJOR supporter of XM, though they say Honda and Toyota may join, but havent done any investing in either company yet.
Listen to some of their streams online and you WILL change your mind about satellite radio. It is simply perfect...what radio should be. Commerical free, wide range of music, uncensored and high quality. (atleast sirius is all that). I'm into trance and after I heard the electronica stations... I was amazed, they were playing songs that I had never heard AND couldnt find on kazaa!
With the new Home2Anywhere Kenwood Sirius tuner, you can move it anywhere you want, since it has docking stations for car and home. Plus the best part is that you can tell it your favorite artist and if it is played on any station, it willl automatically go to that one for you. This is the future of radio.