My guess is since there are tens of thousands of receivers of both kinds out there, that use two different sets of frequencies and decoders, both systems will continue to operate for a long time. Programming and advertising sales may start to merge (Howard Stern may start to appear on both systems for example), and when the satellites near their the end of their life expectancy that one (or the other) receivers will be retired. We'll figure out which one will win out (XM's or Sirius') when they stop selling the other one sometime in the near future.