In two of my cars, one has XM and the other Sirius integrated into the sound systems. To me the audio quality seem the same. I much prefer the Sirius programming (jazz, sport, rock) although the XM programming also pretty good. My son put an after-market add-on XM unit in his car and the sound quality was not good with spotty coverage. In my experience, it seems that the add-on units are not as good as the integrated units.
Good luck,
Reg
Good luck,
Reg