The M5 is a sedan; therefore, I would classify that as a luxury sport sedan. The M6 I would list as a luxury sport coupe. In my mind, its a different class. The Viper is all about performance, whereas the BMWs are ment to compete in luxury as well as performance. The M6 is limited to a 155mph top speed; meanwhile, the Viper goes up to 190. The Viper is still faster.

There isn't a domestic equivalent to the M6 because Cadillac doesn't have a hard top, and neither Lincoln or Cadillac is a serious contender anymore if you ask me. If Chrysler gets the Crossfire, then it might have a domestic equivalent.