Both satellite radio services are good just because it's not normal radio. No commercials for the music stations, no censorship on the rock stations, good reception all the time, and good sports coverage.
Either one is worth it if you hate regular radio, but it depends on what you want to hear. If you like baseball, then obviously XM is for you. If you are into football, then go Sirius. I just bought a Sirius radio about four days ago. Having football and Howard Stern swayed me to Sirius.
I haven't listened for long, but I think the music is essentially about the same. I have XM on my DirecTV, and it's basically the same as Sirius. Both have just about every type of music you can imagine.
If you like news, then you may want XM more after the end of the year if Fox and Sirius can't iron out their contract dispute. Most of the news services will be on XM, and only a few are on Sirius.
XM having a truly portable player is a plus for XM, but I'm sure Sirius will have one soon. XM has more subscribers, but that could be because of the car companies that they have deals with (GM is with XM, the others are for Sirius). I just went on my Sirius forum, and it says their new unit may become portable in the next 6 months.
For you Canucks, Sirius just started service to Canada.
To me, sat. radio is like the satellite TV industry right now. XM is DirecTV with more subscribers, and Dish is Sirius, in second but still provides a good service. So as long as you want commercial free music and some sports news in the car and the home, either one is good.
(FYI - having more channels is bad on sat. radio...it steals bandwidth from the other stations and makes the sound quality worse and worse)