I think I've heard that the chip set needed for Sirius' format is somewhat more complex and expensive than the chip set needed for XM's format. That's why you also see some rock-bottom pricing of $29 (after rebates, etc.) for portable XM tuner units, versus something in the $70 area for Sirius. And it is not just a simple matter of XM doing more subsidizing of the tuner price - there is an actual cost difference.
It may be that the receiver companies didn't really care which one they went with, and just opted for the one with the lowest-cost implementation. That said, XM does have more subscribers at present.
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