if you like driving, go for the petrol. if you like driving, but money is tighter, save your cash now for the extra fuel later and get a mk5 gti.
if its just a car to you, or you do a lot of miles then the GTD will be a great car.
i'd guess if you drive the gtd hard you won't get much better fuel economy than the petrol.
i'm seriously regretting opting for diesel, and also for cancelling my order for a gti.