Whether or not you use a comma, I would interpret your sentences in post #1 to have the second meaning you describe: "I don't drive a Mercedes. But you are right in a way that I drive something in some degree similar or comparable - a BMW." I would be even more certain of that interpretation if you said "No, I don't drive a Mercedes, but I drive a BMW." This implies "and it is equally good."
To express the more neutral "you are wrong, here's the truth" I'd simply say "I don't drive a Mercedes. I drive a BMW" without the "but."
Edit: try replacing the second car with, say, a Volkswagon Beetle:
I don't drive a Mercedes, but a Volkswagon Beetle - this one doesn't sound bad, so I suppose it could go either way. It still makes more sense to me with a BMW.
No, I don't drive a Mercedes, but I drive a VW Beetle: this just sounds silly. The structure implies equality, but that's not how I think of the cars.
I don't drive a Mercedes. I drive a BMW: this sounds perfectly normal, a statement of fact.