Here's a single vote for Sirius.
why?
1) they have more commercial free music stations than xm for a little more/mo. i already hate FM as it is for all the ads on the radio and since i'm already paying for satellite radio i might as well get a completely commercial free solution.
2) equipment. not to get into a which brand is better debate, but personally i like Kenwood's (eXcelon line) more than Sony's or Pioneer's, both of which only sells XM tuners. additionally, at this point in time the sirius tuners are a bit more affordable.
3) availability. Sirius may be slow to the market but at least they (imho based on what i've read and researched) have better technology. and i like the fact that their satellites are lower in orbit. if anyone mentions this, the reason they also have a dozen or so less repeaters in the US is because they don't need as much. The way their satellite works is that two of them are always locked in your position and always overlap each other so the chances of getting a lost signal (dead space) is less.
4) sound quality. no debate here, they both sound good and as far as my ears can tell there's no difference in terms of sq. i've listened to both and it's all and the same. if you want to know how Sirius sounds, log on to their site. They've been broadcasting all their channels live for quite some time now - free.
I'd like to add, chances are if you're getting satellite radio to complement your non-satellite radio capable tuner you're going to be getting either a cassette adapter or an FM tuneable receiver. At this rate you'll definitely not notice the sq differences (if any) anymore. Satellite radio doesn't come close to CD SQ just yet but it is definitely better than regular FM or Cassette. And if you're playing it using those means then SQ is already negligible to start with.
5) marketability. they'll both survive IMHO. their ties with the audio and car manufacturing industry should help them survive. (GMC, Honda, Isuzu, Suzuki for XM ... Ford, Daimler Chrysler, BMW, Lincoln, Mazda, Jaguar, Volvo, Jeep, Dodge and Mercedez-Benz for Sirius)
At this point, the real deciding factors are:
Less (XM-30) vs. More (Sirius-60) advertisement free music channels
Cheaper (XM- less than $10) vs. More expensive (Sirius- less than $13)
Hope this helps a little bit.