That is very cool!I am also facing this choice. I am leaning toward XM in part because they have a great new player, the Pioneer Inno, that can be used not only in the car and at home, but also as a portable with headphones. It also allows you to add your own MP3s like an Ipod and to record songs you like while you are listening to XM (or to program it to record).
I'm going to get it for my birthday!
http://reviews.cnet.com/Pioneer_Inno_XM/4505-7873_7-31824917.html