Sirius has all the major sports except MLB.
XM has MLB.
Music wise, Sirius plays more popular stuff, XM plays the "deep tracks"... The songs on the artists' albums you never really previously heard on the radio. Just depends on what you like. I love Sirius, but I can't say I wasn't kinda jealous riding in my friend's car who had XM and some Green Day that I'd only ever heard on one of my CD's came on the radio :/