Hmmmm BIK...
Based on a 5 door manual version of each with no extras:
20% Tax
R
2016-2017 £1,896
2017-2018 £2,022
GTI
2016-2017 £1,304
2017-2018 £1,413
GTD
2016-2017 £1,177
2017-2018 £1,284
40% Tax
R
2016-2017 £3,792
2017-2018 £4,044
GTI
2016-2017 £2,609
2017-2018 £2,827
GTD
2016-2017 £2,354
2017-2018 £2,568
I make that over a hundred quid a month come next April (at 40%, I'd guess you are a 40% payer if they are giving you a 30k car!) between an R and a GTD. Plus extra lease costs too.
The difference between GTI and GTD is a bit more moot, but if you said the GTI does 40mpg and the GTD does 50mpg.... thats a 25% increase in fuel costs - can be a lot if you do a lot of personal miles too.
Its all by the by for me anyway, we aren't allowed anything over 150g/km emissions in line with EU Co2 targets. GTD or nothing. Am hoping the 2017 target doesn't get noticed or I am driving a bloody Prius next time....*spit*