+1 for CP if how long it will take you to "beat" the machine is an issue at all. An even half way decent player will complete the Stiff-o-Meter in no time, and then end up doing it multiple times in a game. I think that same player would take much much longer to run through all the boxers on CP.
NOTE - SS also has the Spider Mania multiball that some people might consider it's true wizard mode. That's definitely a way harder thing to get to.
I'd rather have CP many times over myself. I think it's a much harder table, and the theme is pretty dang unique, with an awesome toy in the middle. When playing SS for points, I just found myself looping the same ramp repeatedly game after game.