I had a 92 316i a long time ago, i found it drank petrol
If it was my money and i had to buy new for 6K more i'd have the 318d
It should hold its valve more, the 318d does on average 50mpg, not sure what the 316i does as figures are not on the website but the 318i does on average 38mpg (not that bad, way better than my old 316i)
on the negitive side:
diesel is overtaking petrol price wise (why is that?)
cost an extra 150 a year to tax engine.
insurance would be the same..ish
i suppose it depends on the amount of mileage you do and how long you keep the car.