135Ci
Frugality and low tax and so on, don't really add up to that much when we consider depreciation, and the diesels will depreciate more than the king of the hill 35i

(imho)
To make the diesels pay you have to do mega mileage too.
Personally, I think buy a diesel if you want to save money, and buy a cheap older car to do that too. Buying a nearly new expensive car, and getting the diesel over the petrol to save maybe £50 a month, when the car depreciates at £500 a month, seems, err, wrong
Don't take offence, just my 2p
If saving money is important, buy an old 330Cd or something.
If you want a cool car, a 135Ci is the way to go... stunning, rare, hold their value really well (offsets the not so bad, but higher than 123d running costs). As soon as they get near £15,000 I'll be having one with a Birds sourced diff and a remap!
As for remapping a diesel, I'd always get the best diesel you can get first, before resorting to remaps for more power. Remapping hurts insurance, residual value, increases component wear rates, and costs a not insignificant amount to get done. Buying a better model to start with might net you better residuals, more standard kit etc, for a very similar overall running costs point of view
Dave