I have had XM for about a month now. It rocks! I have 18 channels set in my HU and I listen to all of them, depending on the mood. I have NEVER lost signal.
XM Customer service has been outstanding. I wrote the programming director for one of the stations becase a show that they said would be on at midnight wasn't on. The dude emailed me back the next day and apologized. He told me what the problem was and asked if there was anything he could do to make up for my show not being on.
I listen to a lot of the electronica and dance. They have all my favorites and some stuff I haven't heard. I rarely have to change a station because it sucks.
The quality of the music is CD quality. By looking at the "car adaptor" that you have, I am not sure you will get the same quality. Because you are converting from a digital signal to an analog signal, you are going to lose some of the quality. If it were me, I would get a XM (or Sirius) ready HU and a XM (or sirius) digital tuner. Definatly more pure sound.
As for XM vs. Sirius, it is a matter of which HU you like. Sirius is $3 more a month, but totally comercial free. XM is cheaper $9/mo. and some of the channels have commercials. I am a cheap bastard, so I got XM (actually, I like Pioneer)!