XM Versus Sirius
Radio Satellite, not the prettiest of websites, has a comparison between XM and Sirius satellite radio. Living in Canada without a car, I have not experienced either service, but I think knowing the ins and outs of what you are getting before you buy it is probably a good thing and good comparisons are hard to find.
And with about 6.2 million subscribers between the two services, it looks like the technology is doing alright.
If anyone uses either of these technologies, I would love to hear your experience with them.