Petrol.
Anyone that buys the 4 cylinder diesel BMW engines are just uneducated in my opinion. They are clunky, noisy and generally not smooth at all. If you're going for a diesel BMW, get 6 cylinder (which is 2.5ltr onwards if I'm correct?).
This is my opinion. But I feel strongly about it.
There is a lot of engine snobbery on here isn't there?
The x20d isn't a bad choice at all, assuming it's fit for purpose. It's one of the more powerful AND more economical 2.0 diesels out there. To say anyone who buys one is simply under-educated is, quite frankly, incredibly insulting to be honest.
For example, I could have easily made the case for getting a 520d. And, in fact, that is what I had originally specced up when I was in BMW looking to buy my current car. I easily do the mileage to justify it. But, I could also easily afford the 530d, so I went for that. Not everyone will be as lucky as to afford both the extra purchasing expense as well as the extra fuel and maintenance costs. In which case, the 520d makes for an excellent buy. Or the 320d, should that be all you need.
An example of difference in costs, is that my 530d cost me an extra £45 per month on a PCP than the 520d. This cost is basically the difference in depreciation between cars, so is a reasonable number to use regardless of finance arrangement. And at 20k per year (a figure often thrown around these parts as the minimum required to go for a diesel), it's around £42 per month extra to fuel the 530d. So that's almost £90 per month more for the 530d over the 520d, simply in depreciation and fuel (assuming new cars here). That is not nothing when tallying up monthly expenditure.
I don't think anyone would try to tell you that the 6 cylinder diesel is worse than the 4 cylinder diesel. But that is a long way away from claiming that anyone buying the 4 cylinder is uneducated.