Trying to avoid the petrol versus diesel debate, I will give you some genuine feedback about owning a GTD.
I swapped my Ed30 for a GTD. That was a mistake, which I remedied 6 months later when I got rid of it.
Pros-excellent interior (ie standard Mk6), comfortable and a genuine 50mpg (combined).
Cons-poor handling after a GTI, very nose heavy with pronounced understeer eg on roundabouts. No getting away from it, despite 170bhp it is slow (boring) after a GTI. You need to get the 18'' Vancouvers to make it look decent as the standard 17'''s look crap IMHO.
Mine was Carbon Grey and the paint literally fell off the bonnet at the slightest touch. Stone chipping the worst on any car I have owned (though not the GTDs fault as such, but VW paint in general?).
The GTD is typical of 'some' VW products, a triumph of marketing over substance (meaning it's too expensive). Just looking at second hand examples on the VW site the prices are ridiculous. It makes more sense in my opinion to buy the GT TDI and if you want extra performance get it mapped. There are plenty of specced GT TDIs including some with Vancouvers that are cheaper than a poverty spec GTD.
I appreciate that some people have modded their GTDs into great machines, but as standard and after a GTI mine soon left me flat.
Obviously as an 'infrequent' (ok, hardly ever) poster on here I am prepared to be flamed (!!), but having gone through the GTD 'process', I feel well placed to advise others.