I was torn between the 1.5 petrol and the Sd but on test drive didn't really like the gearbox on the petrol as have to have an auto, on the SD it is much better, think I probably would get more MPG out of the 1.5 on the majority of my trips, it is only when you get a decent trip out that the diesel gives great MPG, Having had diesel company cars for the last 15 years I am just used to how they drive, The SD rarely needs to go over 3000rpm pulls like a train very pleased.
My previous 2013 X1 2.0D Xdrive which was a big beast of a car compared to the mini was returning 53mpg all the time, same engine I think but better MPG very odd.