In the real world on performance there's not much in it - sure the GTI is 8mph faster top end (academic) and about a second quicker 0-60 (if you like traffic light grand prix's). You will find people in both camps here.
If "driving" really is your thing and the excitement of faster acceleration and the smoothness and rev-ability of a petrol engine then it's an easy choice - GTI all day long. As good as the diesel is, it's not a true enthusiasts (or petrol heads) car.
The flip side of the coin is however that the GTD is almost as quick in the real world, will return 45mpg all day long (and mid to high fifties on a run), whereas the GTI will be high twenties low thirties (although someone did once get 40+ from a GTI but think he was driving like a granny
). The other thing about the GTD is that it's lazy torque and quite often doesn't need changing down to accelerate , although to be fair to the GTI, it's engine has an almost diesel like torque output as well.
Other things to consider are a) Do you like the red accents in the grill / lights / seats, or prefer the more subtle grey / chrome of the GTD. b) Do you like the GTI alloys or the Negaro units of the GTD (the optional 19" Santiago units are the same on both).
One final thing to consider is that if you really want to cane it, the GTI is the only one with the option of the performance or "PP" pack. Not only does that give you an additional 10bhp and bigger brakes front and rear (with GTI logo calipers), but also the frankly superb electro-mechanical limited slip diff - although this PP option is another £1000 on the basic price.
To be honest you can't go wrong with either - both are fantastic fast, quiet, comfortable, stylish, well equipped motors.
TBH, your next dilemma is the options list where you can easily add £7-8k of options !
