As fotodad knows... I am a sirius fan myself. In most instances Sirius is superior to XM with content, and selection. Although some stations, like the 90's station on Sirius suck. I do feel that sirius has a very strong Rock, Dance, Hip Hop, and dance line up.
Check out some opinions at
www.siriusbackstage.com or xmfan.com (although xmfan.com has been down the last couple of days).
If you go with XM... the commander is a much better option then the roady. Slimmer profile... and thinner. BUT, you must hide away the tuner box in the trunk, or under your seat with the commander. The tuner is built into the roady. Either option you need to install the antenna on your rear deck, or on the trunk.
Sample the music streams on sirius.com or xmradio.com before you do anything.
If you get sirius... go for a new headunit or the Streamer. See my install pic below. If you go with XM, go with the commander.
My install
Commander, antenna, and hideaway tuner box.
crutchfield.com is a great place to buy either.