The Golf GTI has a got a wonderful smooth engine, and yes it's well worth the extra cost in fuel, which isnt much anyway. I've not noticed much different between the Golf and my previous car (leon FR TDI). I think its we do mostly town driving, short journeys, and the GTI warms up quicker than a diesel . (all cars are bad on fuel when the engine is cold).
Btw, I bought via Car wow, and Peter Cooper was the cheapest, so i drive 250 miles or so to collect my car.
I honestly dont think its worth buying nearly new, better to buy new.