With either service you will certainly find situations where you won't get satellite reception.
Both XM and Sirius have terrestial repeaters in major metropolitan areas to give service where you don't get good satellite reception, as well as indoors. How many, and where, these are is fairly private information, and about the only way you will be able to tell if there is coverage that will serve you is to actually use the service.
With regard to satellite location, XM uses geosynchronous satellites over the equater, while Sirius uses 3 satellites in varying orbital positions over the north/central US. At least in theory Sirius should provide better service in more northern locations, however it again depends on your immediate surroundings (deep canyon, heavy tree growths, city buildings, etc. will all interrupt signal). I have travelled all 48 of the lower US states and all bordering Canadian provinces with Sirius on my bike, and rarely had service interruptions. Can't speak for XM because I don't subscribe to it.
Carl