Sirius or XM ?

When I was at Zero Hour last week, the car I rented had XM Satellite radio. I really liked it, and was thinking I should take a look at this satellite radio thing, and see if its right for me. I have some friends that work for Sirius, so they are trying to get me to sign up there. If you have one (or both) what do you recommend and why? (Yes, I know Howard is going to Sirius, but what about the music selection or hardware choices?)