No one's discussed the sound quality of satellite radio, which is wholly disappointing, both for XM and Sirius. By cramming 100+ channels into their limited bandwidth, the Satellite Radio companies have to compress the hell out of all channels. This is fine for talk stations, but the music reproduction is poor - it's akin to a very strong AM radio signal!
What kills me is some people claim satellite radio has CD-quality sound. Sure, it's all-digial, but it's definitely NOT CD-quality. The music stations' bitrates are around 64 kb, which anyone familiar with mp3's knows, this is not even close to CD sound (which is 128 kb and above).
If you have a very basic sound system in your standard car, you'll probably won't notice a difference, since everything will sound muddy. However if you've upgraded your stereo & speakers (or have a nice stock system in your luxury SUV), there's a night-and-day difference. Play a song on XM/Sirius then pop in a CD; the gap is amazing.
That being said, Sirius is the way to go since I've bashed satellite music. It dominates the sports aspect, has all the news/talk you can handle, and is getting Stern in January.