I would personally go with Sirius over xm, I had xm for a while but I began to hear patterns in the music all the time. It was getting to the point were I could predict what the next song would be on any given station... I haven't had that problem with sirius yet...
but here is a link to a cnet review that brakes down there services a little. Like they say if you are a christian rock fan then xm is the way to go, if you like talk stations then it leans towards sirius. Here ya go enjoy
http://reviews.cnet.com/4520-6466_7-...4.html?tag=dir