I have sirius and i think it's great. I do however occasionally lose signal under large overpasses and sometimes on the road between two large trucks.
I sell Sirius, so let me explain some of the difernces.
XM uses re-broadcasts of ClearChannel radio broadcasts. I personally don't have a problem with this if they take out ALL of the commercials. which they say they have, but i haven't listened to it recently.
XM has 2 satellites, and TONS of terrestrial repeating towers, so it's a little better for getting signal on the road.
Sirius uses fresh broadcasts from thier own studios in New York, which is kinda cool, but you still get the repeating of the same popular songs over and over. I thought this wasn't going to be the case. I hate ClearChannel Communications stations because it's so repeatative, but it's really just the nature of radio.
I find some of the programming to be a little better on Sirius, it's a little less mainstream. so you are more likely to hear new artists (i've heard a few that i've complety fallin' in love with) but if all you want to hear is OutKast: Hey Ya, either format can oblige you.
Sirius has more sattelites (3) and much less terrestrial repeaters. sooo.....what this means is better reception on the water than XM. so if you want to get reception on the boat, it's better to get sirius because if you aren't near a repeater, forget it.
Sirius has a NasCar team.