I have Sirius in my car, my wife has XM. I have to say I greatly prefer Sirius' channels over XM's. The music selection and placement of genres is much better than on XM. I have listened to Xm's 80's channel for 45 minutes and never once heard anything I had ever remembered on the radio in the 80's. Decades channels and genre specific channels shold highlight the best of whatever era they are from. I'm fine with a B side or deep cut here and there but not all the time. Sirius channels actually line up almost identicla to my Ipod playlists (80's, Hair bands, classic rock, classic rap, alternative, etc)
That being said, Sirius does overplay some artists and songs. A little new variety woud be nice.