XM has a much broader selection of music, and they play it. Over 2
million tracks in their library, versus 500,000 for Sirius. Sirius
has the NFL games though. I have both and find the sound quality for
XM to be superior and the playlists are much deeper. Sirius has had a
lot of complaints about repetition, and I pretty much agree with it.

XM also has much better hardware. They are at least 18 months ahead
(or more) in their chipsets. Unless one wants to listen to NFL games,
I would choose XM. I think some younger people may prefer some of the
music on Sirius, though -- I can't listen to it, personally.