My wife and I have had Sirius since the beginning and I've used cars that have XM. I Prefer Sirius but that might have more to do with familiarity and not the service. You really can't go wrong with either and it becomes one of those services you look back and say Why did I wait so long and I'm not sure I can do without this service.

If they merge like I hope (I have a ton of Sirius stock) you will have access to both.