Based on my current mileage having a gti over the r has saved me about £400 in fuel in the 8 months I've had the car. I could have saved even more by having a gtd but that would have meant having to fill up from the wrong pump.
You are filling up from the wrong pump (RON95).

With the minimal price differential between the 2, buying a GTI over an R makes no sense to me - if you want fuel economy and 2WD get a GTD and a tuning box. For me the R is worth paying a little extra at the pumps for.
I may be facing a car dilemma soon - applied for a job that would require a 96 mile round trip commute, with that in mind i'd be looking at either:-
1. Running the wife's A1 1.6TDI for the commute and her having the R for her commute, allowin me to use the R on the weekends.
2. Selling the R and the A1 privately, getting a new S3 (if facelift is due) for the wife to commute in and me to use at the weekend, and a crapbox (likely a 5 year old Vauxhall Astra diesel) for me to run up the commute miles in.
3. Doing as per 2 but keeping the R instead of getting an S3.
4. Hope to get a 2 or 3 way car share and keep the cars as they are.