I went with XM due to them being so far ahead of Sirius in technology, I've driven on roads in the middle of nowhere and got a ground signal...
Sirius worries me, they are investing so much money in to "names" and not "content". If Stern does not triple their listener base in the first year, I can see them having major problems and possibly being bought out by XM. I use to love this guy years ago, but he lost it... he just doesn't seem to have it in his heart anymore and I think he is just there for the paycheck now; that doesn't make for good radio.
When I say "names" I mean Martha Stewart, Richard Simmons, Tony Hawk (awful show), Lance Armstrong and Bam... people who you may want to meet, but why in the hell would you want to listen to them?
NFL -vs- MLB : God, baseball was made for the radio... football on the radio? its so much more a visual experience.
Bob Edwards was a great addittion for XM, Opie and Anthony - I have fallen in love with this show, it actually beats Stern when he was at his best.
Music is about the same, depending on your taste is a coin flip... if you love Elvis and Buffet, Sirius might hold an edge.
I had Sirius for a few months due to using a friends car while he was away on a business trip; I just didn't like it as much.
Now XM is not perfect, 152 is just clear channel shows - but the contract is almost up and word is XM is taking it over again (Clear channel had a major stock holding and forced their shows on XM). Playboy radio sucks... and its going away, the channels that costed more was a horrible idea - but they are all now free once playboy is gone.
XM is sure to get more bandwidth soon, with the contract with Homeland Defense to transmit information to troops in emergencies, word has it the are almost guaranteed FCC approval for the bandwidth addittion for ground repeaters.
I think they are getting Hockey, another decent sport on the radio since on TV you miss so much of the visual aspect of being their that the audio feed is all you need.
News... they have all the cable channels, I listen to Fox News a bit and am surprised how well not seeing the broadcast plays out. BBC is great, wish they had all the channels though (rumour that with the canadian deal they may get their new channels)
But, no matter which way you go... its better than AM/FM and you won't want to live without it.