Yeh, the SD is a bit noisey at cruise. But not real bad. My wife has a '96 Chrysler Cirrus. Now that's a noisey car! The road noise at freeway speeds gets annoying real fast. Makes the diesel quiet by comparison.
I've never ridden in a newer gas Benz. Must be whisper quiet on the road.
You just have decide what you want. Gas mileage, or power. I find the turbo-diesel to be a nice compromise. Sufficient power, very comfortable, and a consistant 25mpg no matter how you drive it. Plus I'm one of those crazy ones that just likes diesels. But my next Benz will probably be a gas-powered. I want something like a 560SEL or SEC, or a 560SL. That would be my boulevard cruiser