XM decks aren't XM ready only. Sirius teamed up with pretty much every company to make Sirius plugs for all the XM companies, not sure if they're doing it vice versa. The direct link XM plugs are out now (well some of em: Sony, Pioneer, and Kenwood). The Alpine XM tuner/plug combo actually has a pass-through so in theory you could use both services. XM has much better plug and play units, although Sirius is starting to catch up.
I personally prefer the programming on Sirius better as they have more rap/hiphop stations. Stern won't be on till about a year and a half from now, so it's not really a deciding factor, XM does have O and A right now which is supposedly pretty funny (even more extreme than Stern) although I haven't heard it yet.
NFL is nice on Sirius, but it's only 16 days out of the year. XM is picking up college sports (football and basketball) and has Nascar (2 stations, 1 with the radio stuff that the driver's and commentators actually hear).
XM is also adding the Starbucks channel thing (Hear Music), which is going to mean a lot of money for them, so invest in their stock if you can. Sirius has NPR but XM has Bob Edwards, the main reason why most people listen to NPR.
It costs XM about $100 to activate an account while it costs Sirius about 5 times that. On XM you can cancel your subscription and get a pro-rated price and you can also transfer your account to and from different units. Sirius subscriptions you can cancel, but you don't get any money back so it doesn't matter. You also can't transfer their subscription, so make absolutely sure you want Sirius before signing up for a longer term. Also Sirius has slightly better sound quality (XM shares its bandwidth with OnStar), but it's not really noticable. Sirius also has 3 satalites (while XM only has 2), but that doesn't really mean anything as only 2 are transmitting at any given time.
Anything I missed?