The diesel clatter is only noticeable from inside at low speed for instance looking for parking, traffic jam and generally speeds below 40kmh. Other than that, when you drive at speeds above that, you hardly know you're in a 20d.
Personally, I think 20d cars are more relevant in mix of city/highway driving. The torque comes in handy when you need that sudden the acceleration to overtake someone or hill climb. Of coz if you do highway all the time, the 20i is a great car to hit high revs when you need that top speed. But a BMW purist would tell you that you enjoy highway speeding only in straight 6 engines. That is not to say straight 4 pot isn't great but the refinement is not as good as a straight 6.
Above all, test the car before you make a decision. I made 3 test drives within 2 weeks before I made up my mind on the 320d. But the first test drive made a lasting impression as the sales guy kept insisting that I floor it on an uphill section and there was this sweet long stretch which was enough to hooked me. The 3rd drive, my wife tested it too, and then she insisted to get that car although I was still thinking if I could afford it or not.
And yeah, till today my parents still goad me about driving a truck. I tell you it's a stigma when you're driving diesel cars. People associate it with trucks or old bus without wanting to know the benefits of Diesel engines.
