This is a question I keep asking myself.
Have had a mk6 GTi, Mk7 R and now a mk7.5 Gti PP. I must admit that I really enjoyed the R and it does feel much faster than the GTi. It also sounds better both inside and outside than the GTi. At the end of the PCP, I could not get a manual R so got a manual GTi PP instead. I was a bit disappointed to start with, but that was my own fault because I kept trying to compare the Gti PP to the R. But it is a totally different car to drive and it is not fair to compare it. So, I no longer compare it to the R and I must say that I am really enjoying it. It may not drive or sound like an R but it definitely looks better and feels lighter when driving through the country roads with all those bends to enjoy swinging around.
I think that they are both good hot hatches and have different pros and cons. I will probably be getting the MK8 when it comes out, but the big question is will it be an R or a GTi???