I have XM. The baseball coverage is insane. They have every spring training game (that has the ability to be covered), every MLB regular season game, and a nonstop MLB talk channel which I cant get enough of. A few of the baseball talk guys get a bit annoying but the coverage is great. During the season (and maybe offseason, Im not around at 12EST to listen) theres a fantasy baseball show for an hour every weekday hosted partially by rotowire's jeff erickson and chris liss. XM carries NHL and Nascar (neither of which I care about). The baseball and music is enough for me. In addition, they include a large variety of other sports stations (fox,espn,etc).
Ive heard Sirius which is great as well in the event youd rather here Football games every sunday rather than watch. The DJs on the music stations are a little more talkitive which at times is nice but can get annoying. The stations on both XM and Sirius are pretty similar, I hear XM has more but Im sure youll only listen to a handful of of the music channels. Im pretty sure both have a 24 hour stand up comedy channel which gets pretty halarious on XM. I know I prefer XM for the baseball and the portable XM MyFi radio which allows me to record 5 hours of my favorite music or radio shows.