People often tell you... "buy the diesel, it has better mpg", but when you actually work out the figures it's quite interesting. So...
330d M Sport base price = £36,280.
328i M Sport base price = £32,060 to which you'd have to add £1,660 for the sport auto box that comes as standard on the 330d, making £33,720.
So the 330d M Sport is £2,560 dearer to buy (like for like).
Assuming you do 10,000 miles a year, and using 57.6 mpg (combined) for the 330d vs 44.8 mpg (combined) for the 328i, and the current price of fuel at Tesco of 135.9 p/L for diesel and 131.9 p/L for unleaded (I'm assuming a constant difference in price between the two fuels), then it would cost you an extra £265.85 a year to fill up the 328i.
Divide that into the higher up front cost of the 330d and on fuel costs alone it would take you about 9.5 years to break even.
Of course this is a very basic calculation because I haven't taken into account the differing annual Road Tax, servicing and maintenance costs, insurance, and even things like interest on the money you save on the initial purchase. Still, it's interesting that if you're doing sums just based on mpg then the 330d doesn't start saving you money until nearly ten years after you bought it (if you kept it that long)
What gets me is why diesel in the UK is so much dearer as I assume it's cheaper to refine, transport and store... and does the diesel car really cost BMW about £2,560 more to make?