The dirty secret is that satellite radio sounds worse than FM radio. Sirius sounds like a 64 kpbs mp3 and XM like a 96 kbps mp3. Why are there people who say that satellite radio sounds better? Because they are being paid to come to message boards like this and lie! This is the heaviest volume car audio message board I've found that isn't dedicated to hardcore modifying; the industry has a big stake in prevailing opinion on these pages.
For some very good discussion from some knowledgable people, there are some threads to reiterate my above contention over and over and over and over and over and over again.
That second thread is 29 pages long but you'll get what you need off the first. I recommend reading the entire first thread because you get some knowlegable people writing posts.
Yes, there are dissenters who claim satellite radio sounds good on those threads- these people are lying. The only legitimate reason to prefer satellite radio is trouble with reception of analog FM.