XM in the northwest regions of BC starts to become rather weak and is prone to dropping out quite a bit. Did that test a few weeks back driving with both XM and Sirius ...
I figure the XM satellite is closer to the eastern seaboard high above the equator as a result. Sirius is closer to central north america ... for a footprint.
As a result ... no XM in hawaii. My new Nissan car owners manual states that the XM/Sirius sat option for the car is not available in Hawaii ... Guam
XM have two sats geosynchonous over the equator. Sirius has three sats which orbit around the US. That means that you are more likely to get a signal from sats with Sirius, since they are overhead, and the XM sats are more on the horizon. XM makes this up by having more repeaters. This is why you are more likely to lose Sirius going under overpasses.
from what Ive seen, Sirius is better up north due to the orbiting sats. XM can be gotten as far as the artic circle, but only one sat (so 1/2 the programming, the music), and you have to be stationary, it wont work with the unidirectional car antenna, just the home antenna, since you can aim it better. As far as going south, Ive heard XM comes in perfect till about Belieze, then you have fade outs.