As a diesel fuel injection system engineer, I er on the side of the diesel.
However,
A GTI is a fun car, which can be owned by those who don't mind the running costs and enjoy the car, despite it's day to day cost.
A GTD is a fun car, which can be owned by those who want economy and performance as a trade off, and not to worry about the day to day cost of it.
Basically - GTI = Fun but expensive
- GTD = Fun but cheap
If you knew the engineering that went in to producing the diesel's then some of the reasoning in this thread might change.