I was home visiting my parents this weekend and saw that Dish Network broadcasts all the Sirius programming on their music channels. I have XM in my car. I only listened to Sirius for a couple hours, but it seemed like the selection of Rock was a little better.
The problem with XM is they don't have a channel that plays ACDC, Metallica, etc all the time. There's the Boneyard, but they seem to play more crappy 80's hair bands then anything else (I see Winger at least three times a week
). I listen to Squizz and Ethel most of the time.
You can't really go with the lineups on the XM sites either - they're a tad misleading. The lineup for the Boneyard shows Ozzie, Metallica, ACDC, Van Halen, and some hair bands - but it seems like the hair bands take priority most of the time.
Right now, if I was subscribing again, I'd go Sirius. In fact, if they have another equipment deal like they did recently, I might subscribe now and give my XM stuff to my sister.