I have had both. I started with an XM when I lived in Ontario and switched to a Sirius when I moved North.
The XM satellites are lower in the sky and reception suffers in the far North.
The Sirius I find has better programming than the XM. Pricing is about the same and you can listen to Sirius online when not listening to your radio, (XM was charging to listen online). I believe there are more accessories for the Sirius radios as well, you can get extra in car kits, home kits, boom boxes so you can take it any where.
Oh Yeah, they don't work well at all in the mountains. All that darn granite blocks the signal!!!! I lose my signal when I head into the Yukons St. Elias Mountains on the way to Alaska and I don't get it back until I return from my trip!!
One more thing, Sirius has the NASCAR Channel!!!!!