To me the GTD/GTi comparison boils down to one thing - money.
Not necessarily. We don't do a huge mileage, so fuel consumption isn't that important. However we often do long journeys where fuel range is important to me. This weekend the Golf will do a 600 mile round trip to visit family. It will easily do that on one tank of fuel. It's only a small thing, but having to be bothered with refueling whilst away or en-route is one thing we won't have to think about.
Similarly with my Passat, we take that to France a few times a year. With it's 700+ mile range, it will always get us to where we are going without having to stop and queue for fuel. Having stopped for a sandwich and a pee, we can be back in the car and probably 20 miles further down the Auto Route in the time it would have taken to put more fuel in.