Sirius... and not because of Howard (whom I can't stand)... better programing in the music channels... IMHO...
The equipment is all pretty much the same for Sirius to XM... but Sirius has a synch (or a delay, can't remember which) in the signal so you don't get the stadium affect if two radios are tuned to the same stream.
Sirius seems to work better in the mountains, since the Sats or on a higher inclination than XM... but I have noticed the signal dropping out under overpasses a little more often than before... could be my antenna is getting old.