Been down to my usual vw dealer this morning and sat in the gtd and R, also looked at the new Tiguan and it's nice but not what we need. My jaw hit the floor when the mrs said 'the seats were more comfortable in the R so why don't you get one of those, it'll be better for your back'!!
I thought the seats were more comfortable even though they are the same shape etc but it must be the alcantara cloth that makes them feel more comfortable.
Done some rough figures and the R comes out about 80 a month more than a gtd and about 50 a month more than a gti without including fuel. Called into Audi dealer too to look at an S3 and they didn't even have any!! If I've got green light for an R I may aswell look at an S3 
So there's only £30pm between the Golf and the GTI in PCP terms? Considering your 8k miles PA, i'd expect you to average 45mpg in the GTD, 33mpg in the GTI and 30mpg in the R (all manual). Based on the current fuel prices I get locally of 106.9p/L for petrol, 107.9p/L for diesel and 112.9p/L for RON 99 petrol (Tesco Momentum 99), you're talking:-
£879 pa fueling costs GTD (£73pm)
£1186 pa fueling costs GTI (£99pm)
£1379 pa fueling costs R (£115pm)
All in then, over and above the GTD, the GTI is going to cost you about £70pm more inc car tax, and the R is going to cost you £136pm more than the GTD.
If I were you, i'd get a test drive in both (might be hard to get a test drive in a manual R, but you could always have another look at mine) - you don't want to end up in a GTI and wishing you spent the extra for an R a year down the line. I can't see the R seats being any more supportive Chris - the Alcantara bolsters are probably a bit better at gripping the behind and torso - that may add up to more comfort.
If you are looking to chop the Tiguan in for an Audi, prepare for fun and games - they're not very receptive to haggling locally (Silverlink/Scotswood Road) - it was hard work cutting them down to size for the wife's A1, despite having nothing to chop in.