I have XM in my car. I get an excellent signal as long as I have a view of the Southern sky. I drive one route where there is a high cliff face immediately to the south of the road, and the signal is cut off intermittently there. XM uses geosynchronous satellites, where Sirius uses satellites in elliptical orbits. I haven't used Sirius, but my understanding is that its coverage is not quite as good as XM (although most users wouldn't notice the difference). Sirius is sucking wind financially, while XM is going great guns. Unless something dramatic happens, I think Sirius is going to get forced out of the market. Its customers will probably eat dirt, as the receivers aren't cross-compatible.

The programming packages offered by both services are roughly equal. The deciding factor would be a matter of taste - if one service had a program or host that you just preferred over the other. I bought XM because it was originally about $2 per month cheaper, but now they've raised their rates, so there's no real difference. I don't find the programming on XM any more compelling than the stuff on Sirius, but I'm not a sports fan. XM's got more baseball, but Sirius has more football.

All that said, I can't see any reason why a satellite radio, Sirius or XM, wouldn't work well in an airplane.