XM or Sirius?

I'm looking to buy a satelite radio for an older Cadillac. Which service is a better value, and which has a stereo that would look nice in my Caddy or be very inconspicuous? I would like one that doesn't attach anywhere permanently, that would possibly be portable to my home, office, and other cars. Note: As far as programming mostly I want to listen to good music, and not just mainstream, the occional baseball game and some news. Maybe NPR once in a while.