I'll preface this by saying I've only owned and listened to Sirius, but I've talked to several folks who own XM. From what I can see now, musically, the two are about the same.
Used to be XM had some commercial music stations, but they've since gone 100% commercial free for music stations (like Sirius). They've also raised their prices to match Sirius (XM used to be a buck or two cheaper).
If I was going to decide between the two now, I'd base it on any of the sports or specialty programming the two offer. At this point, Sirius has NFL, college football, they'll be getting NASCAR next year and Howard Stern starts in January. XM has Major League Baseball and they just got the Hockey contract. For a more complete breakdown, I'd check out the Satellite Radio info on crutchfield.com.
I'll usually listen to NFL games and NASCAR races when I'm travelling on the weekends, so I'll stay with Sirius. I love the way they do football games, they broadcast each game on two channels, so you can listen to the home announcers from each team.
Either way you go, I think you'll be happy. The worst thing I about satellite radio for me are the times I'm in a vehicle that doesn't have it. I can't stand commercials when I want to hear music.
Hope that helps,
Jeff