I have both Sirius and XM, and I have to say that XM is overall the better 
bet for music. My favorite single channel is Underground Garage on Sirius, but 
they have a pretty limited playlist, they play a lot of the same stuff over and 
over. XM channels tend to be less restricted, the amount of totally strange 
things I've heard on channels like Deep Tracks is astonishing.  If you hear 
(for example) Cheap Trick on Underground Garage, it's either something from the 
new record or one of their "hits" . If you hear them on Deep Tracks, it's most 
likely to be an obscure track or some alternate or live version of something. 

But either service is great!

Mark