Well, what that post is probably referencing is that generally a diesel will cost more initially on a comparable spec car (i.e. a 140bhp diesel Audi A4 vs a 140bhp petrol Audi A4) from factory. A GT TDI vs a GTI isn't a fair comparison as they're different cars when it comes to power output.
Judging the difference only on fuel economy and fill-up costs makes a diesel look especially attractive – but that’s why you need to consider the differences in purchase price, retained value, servicing costs and even insurance.
At 10,000 miles per year, my above calculation shows that you'd save £800 in fuel. If you bought a diesel and it cost £2400 more, over 3 years (based on 10,000 per year and the same servicing costs, retained value etc) both cars would work out the same in terms of economy.
Do 20,000 miles per year and the diesel wins. Simple really.
Also, and this is just a piece of personal advice and don't really hear it from anyone else, I try to work out a few "if this happened, could I afford it during my ownership"s.
Speculative examples I have looked at before are things outside of your control:
- fuel prices rocketing this time in 6 months
- unexpected servicing
- etc
Good luck. I'm sure you'll find a way to keep the GTI going in the summer months!