I now have both but that may change in the near future. Just an example of coverage: Our Skybox contest has 15 games. Of those 15, XM is covering 4 of them. Sirius covers 10. Only one game, Wisconsin vs Bowling Green, is not covered by either. Glad to have both in situations like those coming up for me. The first three weekends of the college season find me on the road.
If you are a baseball junkie like me, it's XM, which I have. Also, XM carries Big10 football, which, besides ND, is all I care about if I'm on the road on a Saturday. Sirius carries ND, but I plan my weekends around watching the ND games on TV. Side note: My son has Sirius. He tried to listen to the radio broadcast of an ND game while watching it on TV. Didn't work because of the radio delay. The play-by-play was always about a half play behind the live TV action. I'm sure it's the same on XM.
I've got a newer GM vehicle and XM came with it. I enjoy listening to ESPN radio and some of the other sports networks. Like the reception, don't know enough about the fall programing yet.
I'm just in the process of getting Sirius. It might come in handy up in thos North Carolina Mountains someday.
I do think it's interesting that I am able to maintain satelite radio during a heavy rainstorm, while the same will knock out my DirecTV signal.
Consistent with this topic, here is an interesting comparison of the two services: http://www.fortwayne.com/mld/journalgazette/living/15387947.htm