I have the 118i and the main reason I choose for the petrol version was that I do mostly short trips.
My friend has a F20 118d and when I drive his car I always think that it's faster, because of the "push" you get in the beginning of the acceleration.
With the petrol version acceleration feels more gradual and smooth
We have done some sprints from 0-120 km/h and my 118i always wins.
But I still think that the sensation of accelerating is more noticeable in the 118d.
But never forget that the reason why the F20 is such a fun car, is the almost perfect weight distribution, rear wheel drive, steering feel, etc...
It's not just the engine, so switching to a petrol engine is not worth the extra money.