I have both right now. Chances are that I won't be continuing my Sirius subscription when it runs out.
I usually listen to punk or ska. XM has a couple stations that I really like, while Sirius' only mention of punk is in their description of Faction: "Rock, punk, hip-hop and more, especially for action sports fans."
That pisses me off. If I wanted to listen to hip-hop or generic rock, there are plenty of other stations for it.
If I'm listening to satellite, it's probably tuned to XM channel 53. Sometimes 42.