I love my XM
I have had my XM for almost 2 years. I love it, hardly ever listen to cds anymore and NEVER listen to regular radio.
XM is cheaper per month
Sirius is more expensive but they have more "commercial free" channels.
XM's commercials are maybe one every five or six songs, on the stations that have them. Check out the websites, compare and then make your decision.
XM uses two geostationary satellites (Rock and Roll) as well as terrestrial repeaters and such to broadcast.
Sirius uses three geo-synchronous satellites (figure eight above the US). Not sure if they use ground based repeaters/antennas or not though.
My XM works GREAT, I had it in the midwest (MO and KS, and drove from there to Vegas, only loss was deep in mountain passes, or some underpasses, but only for a split second. I have lost signal in underground parking garages as well. Between CA and Vegas there are two spots where I lose signal for a brief second.
Just my 10 cents!