sirius is SO much better. we had an XM reciever for a long time in one of our demo kitchens at work, and it was fun, but the channel selection on sirius is much better, imo. i couldn't find a station that i really identified with on XM, but the car audio department is right next to us in the store and they usually play Sirius, and i hear songs that i love all the time on it.
the monthly price isn't that much (like $13, i think) and if you're in the car a lot or fly often, i think it's worth the price, particularly if you get one of the recievers that's multifunction and can go with you all day, not just in your car.