Sirius has Howard Stern, NBA, NFL, Nascar, NHL, NCAA basketball, playboy channel, maxim channel, blue collar comedy channel, a bunch of other specialty channels. It has like 100 music channels that are commercial free, i've heard XM has commercials on their music channels (I don't know cause I don't own XM).
But honestly it doesn't really matter which you get, cause once the companies merge you will have everything. All existing subscriptions will be honored.