They both use different compression technology so the will only have some content available on both platforms beginning in September. I have Sirius and Howard Stern interviewed Mel Karmazan, CEO of Sirius/XM mentioned that the two platforms will be available for the next 8-10 years to allievieate any confusion from those that have an XM or Sirius stereo in their cars or homes. So it's as if they are really still 2 seperate companies from a content standpoint. Sirius has everything except baseball (which I would love, but MLB gameday audio has me covered for a fraction of the cost online). If you want the best overall content I don't think XM comes close to Sirius, but I am biased and haven't listened to XM for more than a few weeks over several years. If it's specifically baseball coverage you're after, the $12.95/month subscription for XM is probably worth it, but it sounds as if it's similar to Sirius where you get one channel for MLB coverage, but not the actual games themselves online (Sirius offers NFL and NBA). Again you could supplement that with the MLB gameday audio online for about $13/year.