XM hands down because of MLB and NHL
NFL and its 16 game season is best watched on TV. (I don't know too many people that have a football parties and listen to NFL games on the radio) Also, I'd rather pay for Sunday Ticket when it comes to NFL than pay for a Sirius subscription.
Baseball, with it's 162 games, and NHL with 82 games makes XM the number 1 satellite radio sports provider to me. Also, call me old fashion, but baseball translates so well to radio especially when you listen to good broadcasters. NHL, if you can't make it to the actual game, is much more enjoyable to listen to on the radio in my opinion that watching on television.
XM is losing Nascar but I find myself wanting to watch a NASCAR race more than listen to them. Since XM has exclusive NHL and MLB contracts as well as ACC, Big Ten, etc... for college and the same ESPN radio, Sporting News Radio, and Fox Sports radio that Sirius has, I truly think XM wins out when it comes to sports.