It's frightening the amount of topics on this forum that obsess about a few measly mpg. The single biggest hit on your pocket is the depreciation/loan repayments. A few mpg when doing average ish miles is a penny in the ocean.
I still say the GTD should only be considered if doing intergalactic mileage. Some will disagree. Enjoy your GTI imo.
I agree with most of what you are saying - depreciation is by far the biggest hit on the pocket, which is why I went for the R and disregarded the modest extra fuelling costs over the GTI, I feel that it offers a lot more than the GTI for what amounts to a 4mpg difference to me and my driving style (albeit with dearer fuel). Personal circumstances changed that allowed me to buy my R outright and not have 1/3 of the monthly cost of ownership going on PCP interest, my R is the cheapest VW i've had on a monthly basis for many years as I was previously PCPing.
I would worry more about the mpg differences between the the GTD and the GTI than I would between the GTI and the R if you've got at least a 15 mile commute on your hands to get the GTD warm and efficient.
But, when the GTD is cheaper than the GTI (just), has better residuals (bringing down the monthly cost of depreciation) and costs less to run in fuel and tax, that monthly difference is significant even on average mileage. For some it might push the cost of a GTI just outside what they're willing to pay a month to run a car.
At the end of the day you've got to weigh up monthly savings vs fun factor. Will that GTI (or R) be worth the extra cost for the extra driving enjoyment it may or may not derive? That's a completely individual decision.
You've also got to remember that when the first of us got our hands on a MK7 GTD or GTI, fuel was 20% dearer than it is currently - which really opened up the GTD's savings. I'd have to think hard about buying and running an R right now if it was pushing £1.50 a litre to fuel it (currently paying £1.139 a litre for Tesco Momentum), back in October 13, I was paying out about £1.38 a litre for diesel, and premium unleaded would've been about £1.48 a litre.