Traction, and the elimination of tramping are my main motivations for moving from a GTD to an R. Making the change from diesel to petrol may make me wince at the pumps, but the differences between mpg figures for GTI and R aren't worth worrying about (R is 15% thirstier), nor is the difference in road tax. Insurance differences? My GTD costs more to insure on the comparison sites than an R quote right now.
Between tax and lower mpg, an R will cost £33 a month more to run than a GTI for 10k miles pa if both cars achieve 70% of their respective combined mpg figure. Well worth it for my circumstances, the GTI just didn't seem a big enough jump over the GTD to be worthwhile, and traction issues would remain if it was shod with Bridgestones.