im in the same shoes you are.
Wanted the GT TDI but then fallen in love with the GTI.
And now my heart says GTi and my brain says go for diesel (economy)........
But i know if i will get the GT TDI then my heart will look after the GTI and i wont enjoy it.
Il go for it and we shell see, im doing 10 - 12k a year and fuel diference of 600 pounds from the TDI
So if that much costs to enjoy my car and have fun the sod it .
Less beer more miles
and budget wise, maybe for the winter it will drop but you need around 6k mark to find a "good " one.
Don't forget to check the rubber