XM has more music channels, and the sound quality for music is better. Sirius has more news and talk stations, but sound quality for music channels is reduced to make room. XM has Opie and Anthony and Major League Baseball. Sirius will have Stern and the NFL in 2006. Sirius focuses more on popular music, XM plays more obscure stuff.
Sirius has better coverage in Cuba, Costa Rica, Mexico, Canada, and Alaska, should you ever decide to go there. Otherwise, XM can be picked up anywhere south of Anchorage and most of the northern 2/3 of Mexico.
Im not a fan of the Jensen equipment Sirius sells, otherwise the others (Kenwood, Panasonic) are great. What tuner are they offering for $150? My guess is the Jensen -- avoid it.
Sirius is $13. XM is $10. Both offer commercial-free music. I hate when companies have my credit card numbers, and XM is the only one that does paper invoice billing. Both of them offer discounts for pre-paid multi-year service and multiple radios (I have two, XM Skyfi and XM Roady).
I listen to music, ABC News, Fox News, stand-up comedy, local traffic, and MSNBC, which both XM and Sirius have. But XM won me over with a cheaper monthly fee, more music channels, and better equipment.
XM also lacks NPR. I dont know if Sirius has local traffic and weather yet.
http://www.xmfan.com/
-Jesda