Sirius XM audio is highly compressed, in the fact that they run all music through a pro-audio compressor/limiter mechanism (I’m sure this is to reduce the amount of data required, since it’s all based on mp3 type audio compression and they need to cram as much audio channels from satellite as possible). The dynamic range is severely limited when compared to CD. The sound is overall somewhat clean, but separation of instruments is not all together that great, and it can sound somewhat harsh. I have not listened to FM in the last 6 years, but the dynamic range is much better. Though, FM has more of a tube-like sound to it - not as crisp as satellite radio. I stopped listening to FM because of lack of local stations with good music and ungodly amount of advertisements and DJ garbage.
Surprising Audio Quality
I have been listening to radio via satellite for a couple of years. I have the Burmeister system in my car....just happened to switch to regular FM radio (kbaq in Arizona) and shocked to hear it much better in audiophile terms than is the fancy satellite.... interesting....