Let me just say that I was unsure to go with XM or Sirius, being that I could get both for use with my AI-NET 7995 Alpine. I tried to ask around as much as I could and even did the free online 3 day trail of each. In the end I went with Sirius, and I feel for all the types of music I like ( mainstream and defintely non-mainstream) it is great. One second a trance song I like is on Area 33, next second I'm listening to a new rap song I like on Shady 45. They also had Korn on Octane as hosts talking about their influences and playing what they liked ( Mr.Bungle!)
I do admit, the quality is not what I expected, but the bass sounds great with my Alpine M501 sub amp and RE SE 12" in a 1.3cf box. The highs and mids (Diamond Audio D6 and Cadence q4000) can go from decent to almost unacceptable depending on the song and station, but overall the service provides me with non-stop entertainment (I can always find something on I like, unlike regular radio!)
In terms of sound quality and songs, I have heard it both ways, that one or the other is better, I can say the bass and playlist on Sirius is really good and I would take that over just played out tracks with good treble any day.