the long short answer
I never owned an XM unit but a close friend of mine had one and it was terrible - reason being the reception. He has since switched over to Sirius.
Both services (XM and Sirius) need line-of-sight for reception. Both services have repeaters (ground-based towers) but that's mainly only in urban areas where skyscrapers and tunnels would prevent any signal reception. BigSwede mentioned earlier that Sirius doesn't have as many repeaters. Not sure where that idea came from - as I live in North Jersey and have no problem at all with reception on my Sirius unit at home or whenever I go into NYC. The only time I'll really lose reception is going through long tunnels and such.
Something else to remember when purchasing the units. If you buy what's called "plug-and-play" (this is the modular type that you can listen to in the car, then bring inside your house with you - these generally sit on your dashboard), you'll likely have to use an FM modulator (usually included with a car-kit) to hear the music through your factory radio - unless you have a newer car with an AUX jack for your radio. Problem with this is that you're going to lose out on one of the benefits of satellite radio, which is CD-quality, digital music. Obviously the drawback of replacing your existing car radio with a satellite-ready unit is the cost of installation and the fact that you can't bring it inside your house.
Once Howard Stern signed with Sirius I bought a head unit to replace the one in my car. I don't care to have the plug-and-play unit because I wanted the better sound in the car, and to be honest, I don't really listen to much radio at home anyway (except when listening to the Yankees lose :frustrate ).
Pros:
Nicer head-unit than factory radio
Full advantage of satellite radio capability (cd-quality sound)
More security (head unit has detachable face, plus you don't have something sitting on your dashboard in case you live in a higher-crime area)
Cons:
Higher equipment and installation costs
You can't bring the unit inside with you to listen to satellite radio indoors
As far as channel selection - both are very similar in their lineups with between 60-70 music channels, and another 100-120 talk channels. On Sirius I love Alt Nation (for alt rock), Octane (hard rock) and Area 63 (trance and house music). For sports Sirius has exlusive contracts with the NFL, NBA, and NHL (who got the sh*t end of that deal?), and some college hoops. I believe XM has exlusive rights to MLB broadcasts but I'm not positive.
Long story short - both services are pretty similar as far as the majority of their content and pricing as well. Sirius has 3 satellites while XM has 2 so Sirius has better reception without having to rely on repeater towers. If MLB games are the deal-breaker - go with XM cause everything else is pretty similar. Me - I would've purchased whichever service Stern went to, as that pretty much made the decision for me.
Just ask questions, do your research, and don't touch any AAS/PH/Satellite radio till you're 21. :run:
BP