I think the question boils down to: what does your hardware currently support? If you're looking to get a new HU anyway, then it unfortunately becomes a business question: which company is still going to be around this time next year? Neither Sirius or XM are in good financial shape, and while XM has the market share, I've heard a lot of reports that they may not make it past the end of the year. But I'm not so sure on that, so don't hold me to it (sorry). While I think that sattelite radio is a great idea, I can't help but think that it's going to go the way of sattelite phones -- expensive to get started; costly to own at first, then relatively easy/cheap to own; but ultimately unprofitable for the founding company. Now, I admit that putting a single XM/Sirious sattelite up is cake compared to the 30 or so for the phones, but the costs are still very high (no pun intended).
Getting back to my point (sorry to get O/T) -- XM is most likely to survive in the long run because of current market share, but that's not for certain.