I originaly selected Sirius based on reading about their technology. The way they use the satellites, XM had more spotty coverage and had to suplement it with terrestrial retransmitters.
This was a few years ago, when Sirus just started up and XM already operated a few months. These differences may not exist anymore - both services matured.
In any case, coverage of Sirius works well for me, in US and Canada (Ont. & QC).
Then, I like some of Sirius programs - in particular, I listen a lot to BBC News, not available on XM.
Currently, I have a wired-in receiver in my truck and another one which I move between a home cradle and an automotive cradle installed in my K'LT.
XM had about a year of headstart and managed to snag agreements with major automotive manufacturers - this is why you see many more XM receivers in use.
Seems like your choice may be based on programming.