Having had both services, I really don't notice a lot of difference between them except for exclusive sports programming that one may have that the other doesn't.  And of course Howard Stern.

Currently I'm with Sirius and they are ok, though I think I actually favor at least one of the music channels on XM more than I do Sirius.  Make that two now that I think about it.  (XM's Flight 26 is favorable to me over the Pulse on Sirius, and I liked the XM Short Cuts on Highway 16 each week).

Sirius irritated me though by not coming through -- as they promised -- on waiving the re-activation fee when I came back after a year away.  They also bill quarterly at a minimum and I'd prefer monthly even though quarterly should offer a discount.

XM had offered me about 4 months free time when I called to drop them and I just didn't have it in me to keep them as I can XM channels for free on DirecTV and/or via AOL instant messenger (for the channels I want to hear).  Not so with Sirius where I'd have to pay for their content one way or the other.

I would really love to have my old xm2Go radio work with Sirius though.  Much stronger FM broadcast ability on it, and totally portable.  The Sirius Sportster Reply is ok at best.  The FM broadcaster on it sucks rocks and is so totally weak (thanks for nothing FCC!) that it may as well not even be there.  I've also had issues with both signals getting blocked by trees on some of the roads I drive on (rural areas).  No problems on the highway but for one spot under a big over-pass.

Anyway, unless you were looking for Stern, or NASCAR, personally I'd say stick with XM.  Less hassle (though a local Radio Shack should have the cards for you, and Wal-Mart might as well).