I've owned a MKV Golf GTI and I now own a MK6 Golf R.
I've also driven a few MK6 Golf GTI's a few times.
I can honestly say that there is NO WAY that I'd sell a MKV or MK6 Golf GTI to buy a Golf R, unless the MKV Golf GTI was so old that had to or wanted to get a new car.
I can also honestly say that if the MK6 Golf GTI was available in Rising Blue, that I would have bought that instead of the Golf R. AWD is great for ****s and giggles from the traffic lights, but other than that, the disadvantages outweigh the advantages for me (pun DEFINITELY intended). Take away the AWD, and the differences between a GTI and R are not that big. Don't lecture me on this, I know the differences as well as anyone else on here since I've detailed them numerous times in several threads.
You need to figure out what matters to you. Drag racing? Circuit racing? Everyday road driving?
Being AWD, the R will be better for standing start acceleration, i.e. Drag Racing. Both the GTI and R are as capable as each other in circuit racing once you change the brakes and suspension and upgrade the power - one has AWD but has a weight disadvantage, one is FWD but has a weight advantage - but even in the wet around Eastern Creek, the difference is small. Everyday road driving, I'd say that unmodified, the GTI wins here. The R feel sluggish and slow due to it's added weight and lack of low down torque due to the K03...
But again, there is a few hundred pages of this info from numerous people in the GTI vs R thread, which is what this thread was always going to turn into. Even S3 and TT being in the title doesn't change that. At a very simple level, S3 = R. TT = GTI. TT-S = R.