That's it just the MPG and that's the only factor to consider.
The GTi i priced up is 26.5k ish, if the R comes in somewhere in the 28's i'll probably go for that.
If you've added about £4k of options to the GTI, then you'll probably have a big list to add onto the £28.5k (hopefully!) of the Golf R - which will take it to £32-33k

On another note... I test drove a GTD for about 3 hours at the weekend and was very surprised by it! It was quiet, quick and very nice to drive. If I did more miles I'd certainly consider one, but currently I barely do 7k a year. The GTD was torquier, but the GTI was quicker - although I wouldn't say by much! I just prefer getting the noise and feeling of thrashing a petrol car... which you can't really do with diesel.