Sirius subs are more prone to dropouts as their satellites are not geostationary like XMs.
Not exactly.
There are complex sets of conflicting benefits and weaknesses of both geostationary and elliptically orbiting satellites.
One advantage of using several satellites in elliptical orbits is that they can cover a much larger area; while geostationary satellites lose signal strength at an exponential rate the farther north one gets from the equator. This is the reason that it is next to impossible to get an XM signal in Canada without terrestrial repeaters, while a Sirius signal is not limited to a range of latitudes at all. (Both are subject to a similar range of longitudes.)
On the other hand, one advantage of using geostationary is that the line-of-sight never changes. If you have a signal and an unobstructed path to the satellite, you're golden. Even though Sirius' elliptically orbiting satellite network ensures that there are always 2 satellites above 60 degrees altitude no matter where you are in North America, they could potentially be _anywhere_ in azimuth. For instance, if both are above 60 degrees, but one is behind a mountain to your west and the other behind a mountain to your north, you could have a dropout.
Therefore, depending on the circumstances of your particular environment, one could outperform the other. With the increasing network of terrestrial repeaters, the issue is becoming increasingly moot anyway.