I have Sirius in our X3 and my Honda S2000 (I also had it in my previous Audi) and get XM through my DirecTV satellite subscription. I think the real choice here is what *you* want to listen to and which station does a better job of delivering that specific programming.

FWIW I don't listen to any form of talk radio or sports - strictly music and I find that the playlists on Sirius tend to be fairly short so you will get gobs of repeats if you listen to it a lot.

XM seems to fit my own programming tastes a little better but I never would have known that until DirecTV piped their channels into my house as a part of my TV subscriptions.