Here is something else to think about before buying, you may not care about this but I do:

XM is owned by ClearChannel. ClearChannel is a HUGE world wide media network that owns like 70% of the radio stations around the country. They are the reason why you can drive clear across the country and listen to the same 12 songs over and over. I personally have a problem with them because they farked up the radio industry as a career choice for me, its a story for another time. Basically CC is considered by many as "evil"

Sirius is independently owned.

Meh, some people don’t give a crap about this, but just thought I'd throw it out.