I've got both also. I plan to continue Sirius in my car, but keep XM in my wife's car.
A lot of the reason I got Sirius was for Radio Margaritaville, I'm a big Buffett fan. I also tried the online demo for a few days and decided I liked the rock stations a lot better. I think XM is terrible for classic rock - it's all either too soft (Dooby Brothers and Moody Blues 24/7) or too hard. XM has more country stations (also a big country fan), but I found them to be repetitive pretty quick, and actually spent more time listening to a Nashville FM station broadcast on XM (Ch 164 I think). I'