I have XM. I actually have the Delphi Roady 2 and use the built-in XM to FM converter and listen through the car speakers. My friends have noted the excellent sound quality in my car before they knew I got XM (I put sort of them to the test

). I'm telling you, I love it, and it really is everything they advertise it as! They also give you XM Radio Online for free. Cost-wise it's about 10 dollars a month if you sign up for a 5 year plan (the parental unit wouldn't allow that

) and they'll refund whatever you haven't used if you cancel early. I don't think there's a penalty for cancelling early, but you might want to check that out. I signed up over the phone since I had to pay by check, they mentioned some sort of three month at-a-time plan when I told them I wanted to go with the yearly so I went with that since it came out to be the same per month (it was one of those situations where you say to the customer rep, yeah, whatever's best). Anyways, they were very friendly, he even gave me the online activation fee instead of the over-the-phone fee since I had previously tried to sign up online but couldn't because I wasn't using a credit card. As for the radio itself, I'd say go with something portable, just because you can switch it around between cars and won't spend a fortune ($300+) on a permanent installation. The Dephi Roady 2 is very small, about the size of an audio cassette except a little thicker, sound quality is great, you can also use headphones which work great for family trips. Already I have switched it between my car and our other family's two for road trips and it's really convenient. Definately go with XM. I also think the channel lineup is a bit easier to follow, besides the fact that they have more music than Sirius. Sirius is more for news/talk.
EDIT: The Dephi Roady 2 is currently going for $50 at Circuit City. Supposedly it's on sale, but it's been that price for about 6 months. I bought it just a few weeks before they dropped the price from $100, grrr...anyways, I'm a bit P.O.'d that I left it in the garage for a weekend when I got my new car and dirt got behind the screen, so the screen has some very dark splotches. Other than that, it's not too bad. The mount wasn't working out too well, so I just leave it in a little shelf above the cupholders on a rubber jar opener mat thing to keep it from sliding around.