Ive tried both. XM for ~7 months and Sirius for around ~13 months now. Sirius is in my opinion the better option. Either company is better then regular FM radio, but I like Sirius music stations much more then I did for XM. With XMs, I found myself always switching channels like you do with FM. With FM, its becuase of the commercials, XM its becuase of too vague of categories.
Sirius has Stern, which I personally never listen too, they also have every NFL game and its broadcast for both teams, IE 2 channels for every game. Sirius also has NBA and NHL I believe. Not a hockey fan myself.
XM does have MLB, which is not a advantage to me personally.
Throw out all the extra stuff (stern, sports,etc) Sirius just has better music channels in my opinion.