It's a more complete car. And frankly, a better car.
Here you go again making a subjective statement as if it's an objective one. I'm not sure you're understanding this but it's really damaging your argument here.
You could say something like "I lean toward luxury/comfort and I'm not a performance geek, tech geek or environment geek, and that's why I like the S550 more than the Model S." That would be totally fine and I'd have no problem with that.
But if you say, "The S550 is clearly the better car" as an objective statement, then I don't see how you're going to defend that since on many important levels the Model S is better (performance, handling, tech, zero emissions, software updates, user interface, etc) and to more and more people they're choosing the Model S over the S550 because it's more appealing and the better car for them.