Over the last eight or ten years we have had several vehicles with Sirius radios and one with XM all with roof mounted antennae. I have been all across the South, Southwest, and mountain regions of New Mexico and Colorado and could tell little difference between the two services but Sirius has always seemed the most reliable. Both were/are subject to the expected coverage dropouts in tunnels, under bridges, and in narrow canyons. One unexpected dropout occurred/occurs with the older units. Our home is near the local "signal hill" — the transmitting towers for four television stations and a plethora of other RF transmitters including cell phones, AM and FM radio, police and fire radio, etc. The XM and the older Sirius units were/are reliably blanked out within ½ mile or better of Signal Hill. The Sirius units in our 2013 AI and 2015 VW seem relatively impervious to the Signal Hill blackout.
Since all of the vehicles use roof mounted "shark fin", or in the case of the 2013 AI a roof mounted whip, antenna I can only attribute the different sensitivity to the massive RF output from Signal Hill to improvements in the electronics of the later model Sirius receivers. But that is strictly anecdotal evidence and certainly not scientific.
Make errors — otherwise the Great Spirit realizes
you have finished your purpose on earth.
— Navajo saying