Welcome benjyyy!
I've had my current Mk5 GTI for 8 years now, and I fully intend on keeping my Mk7 for the same, if not longer if I can. I totally see the appeal of the GTD (almost GTI looks but with lower running costs), but at the end of the day it's not a GTI, and I don't mean that in a derogatory manner! I know there are residuals to take into account as well as fuel, tax and insurance, but all of that is completely negated by the fact that I'll be paying mine off in 3 years. That leaves me at least 5 years of debt-free motoring, so saving £500 a year is not high on my agenda.