I have had both and prefer xm over Sirius overall. I like Sirius programing slightly over xm, but there playlists seem to repeat far more than xm. Sirius also had a lot more signal drop outs and fade aways where the xm signal was strong.
In the same car with the same system and same antenna location, the xm sounds better than Sirius.
Another vote for Sirius. Had XM in the truck and Sirius through Dish Network. Cancelled XM because the programming had no variety. The Sirius through Dish rocks...now wanting to get that for the road. That playback unit looks awesome.