Maineiac, I'm pretty sure what you said is true. That's why units like the Sirius S50 are basically iPods when in portable mode.
Although I intended only to subscribe to Sirius because of Howard Stern, I did read a lot of forum threads addressing both XM and Sirius. I've read about reception problems for either service and have even found threads where people thought Sirius reception was better than XM. My conclusion is that a lot of that could depend on where you are in the country. Meaning that your proximity to the satellite in your reception zone could be a major factor whether you get better reception with XM or Sirius.
I also wonder if satellite feed is handed off from one satellite to another within ones zone. The reason I say that is because every day around 11:30am my reception at work becomes poor and stays that way until midafternoon. Morning and evening reception at work is good, but the hours near lunch they aren't. I wonder if I'm getting my signal from a different satellite during those hours.
Kind of odd because I never have this problem with DirecTV. DirecTV gives you exact coordinates to line up your dish and once you lock in a good signal it doesn't seem to waver. However, with Sirius I'm only told what zone I'm in based on my location and that I need to orient my antenna east or southeast. Pretty vague.
Have you investigated any satellite radio chat forums? You may be able to glean some useful information there pertinant to your location and application. Luckily, the Sirius versus XM arguments are pretty tame and not like solid state vs. tubes, or the absolutely volcanic Valentine One vs. Escort rage sessions.
