honestly, i'd say sign up for sirius' free online trial thing. sample a bit of their programming. i think xm does the same thing. it's all based on what you are looking for. depending on certain major sports (nfl, mlb, nhl, etc.) and if you're looking for personality driven programming. in my experience, xm is more focused on the music, where as sirius is more focused on getting famous people in their shows.
it's really all just a preference thing.