We've had XM for about 2 years now, and it has been wonderfull on the 1,200 mile trips down to Atlanta. I tend to listen to more talk radio, especialy at night. Although I still miss Art Bell in the mornings, I don't think he is on either right now. The only problem we have up here in New Hampshire, is that we tend to lose the signal a bit easier than anywhere else I have been to. It fades in an out for a few seconds at two locations as we head into work. I don't know if it is the same with sirius or not. Although we also live in a cell phone dead zone. We call our part of New Hampshire the technology free zone.
Sirius just reciently hit the majic 1 million subscribers mark, so it looks like they will be viable. There was some question whether they would make it. Sirius used to have a $500.00 lifetime subscriber deal, which is worth it if you want to go.
As for choosing which one to go with? I would look at the channels offered, as well as for what type of radio you want (if you are putting in a new radio). We prefer alpine radios, they come XM ready, so that's what we got.
-Copperhead