I have both.. XM sucks ass and can't wait until my subscription is up. XM rock stations plays horrible music.. It's almost as if you have a CD, there are 1-2 hits on the CD and XM plays the rest of the songs rather than the hits..
Octane, on sirius, is a station that plays music like the blitz or the wazu non-stop, or if you like heavier rock, the station hard attack is pretty good too.