I still listen to the free stuff. Hell, i don't even pay for cable tv, I'm not ready to pay for radio just yet. (How long til crackheads learn how to hook up free XM?)
I think the decision between which two depends on the specific channels available. I like Howard Stern (wish they hadn't replaced him with this shitty new morning guy in Chicago) so I'd lean toward Sirius. NFL games are nice, but I'm never away from a tv on Sunday mornings. But XM has my boy Tony Kornheiser's show. So I'd have to find some other deciding factor personally.
Do you have a good idea of who specifically your mom would wanna listen to? Doesn't Oprah even have a channel on Xm or something? or maybe she just owns the satellite they use and the Earth it orbits around.