Like many I weighed up the GTI/GTD.
No doubt the GTD will be cheaper to run, but even doing 15-20k miles per year I've still gone for the GTI, accepting I'm paying for the 'feel' of the GTI more than any real world performance gain.
For me it was about the ability of the car to make me grin (not saying the GTD won't - just not as much!), the wide power band, and the red themed interior which makes the GTI feel that little bit sporty & special inside.
On the subject or a Golf RD - I can't see it, surely easier to introduce a GTD PP first?