What kind of driving? Motorway only and on a steady cruise you'll be 50mpg ish on the early 120d, little better on the newer engine. Can't see why 40mpg wouldn't be achievable on a similar run in the petrol car.
Town or mixed driving you'll see a similar difference between the cars, the more town type driving you put into the mix, particularly when the car is cold the more the average economy will drop by.
I've only ever really run the 118/120d on fairly lengthy motorway drives but a friends 116d will return mid 30's to low 40's with a fairly high mix of town driving (say 2/3 urban).
Town or mixed driving you'll see a similar difference between the cars, the more town type driving you put into the mix, particularly when the car is cold the more the average economy will drop by.
I've only ever really run the 118/120d on fairly lengthy motorway drives but a friends 116d will return mid 30's to low 40's with a fairly high mix of town driving (say 2/3 urban).

Very torn because I would love an E87 M-Sport but unfortunately with my current job, fuel economy still matters somewhat.

