I should say I had both ... I prefered XM of the two, much better comedy and longer loops. Since there is a war pendind here ... let me just say GM always wins the war ... look at D* with 13.2M subs.
I bought Sirius for my husband about a year ago when he had to travel cross country. I enjoyed it so much that I wanted satellite radio myself. I subscribed to XM because at that time Sirius did not have a PNP tuner. I switched to Sirius recently because I thought that the music selection on Sirius was better, but XM has a better selection of news channels (although Sirius plays Air America while XM doesn't carry the complete programming). I think the Roady is far superior to the Sirius PNP tuners. I purchased the Audiovox PNP tuner from Best Buy. It's bulkier and the docking cradle makes the unit much bigger. Also there is a bad connection because every time the unit moves the slightest bit the tuner powers off and then back on. Due to the size of the unit I don't really have a place to mount it in my Santa Fe, but the Roady was a perfect fit. I would love to know if Sirius will be coming out with a PNP tuner that does not require the annoying docking cradle.