I've driven the Mk7 GTI, and (obviously) not the R, but I'm still left wanting the R. Let me explain.
The new GTI is a remarkable car. It's very solid, handles well, makes bumps disappear, and accelerates quite nicely. If I couldn't spend more than $30K on a car, it'd be sitting in front of my house right now. But I can afford the $40K which VW is asking for the first 500 R's. I like the fact that the R might not feel quite as "lively" as the GTI, while it accelerates much faster or holds all 4 wheels on the tarmac as it goes through a corner at higher speeds than the GTI. There's a place for the types of cars that require you to throw everything you have into hustling it around corners as fast as you can--cars like the GTI and the STI--and as much as I enjoy that type of car very much, I've grown even fonder of cars which can move even faster through a curve but do so effortlessly and still leave you grinning afterwards. And from virtually all the reports which I've read about the R, that is precisely its forte. But that's only in the dry. In the rain and especially in the snow, review after review have praised how amazing the Golf R performs under those conditions. For one who on occasion watches his wife dive off on snow covered streets, that alone would make me willing to pay a premium over the GTI.
But there's more to why I've been waiting for well over a year to be able to buy the new Golf R, and not a GTI or an STI. To me, the GTI just looks a shade too gimmicky for my tastes. The red striping inside and out; those big plastic slits on the sides of the grille; they all say "hey look at me" in a way that Subaru only further magnifies to almost cartoonish levels. The R seems to have been designed to coolly say "why yes, I'm fast" with the reserve that only comes with confidence--substituting larger, functional, air ducts for the GTI's plastic slits, and deleting it's front wheel drive brethren's bright red striping, stitching, and lighting for silver accents, and blue and white illumination. In short, the R is a very hot hatchback without the "boy racer" looks and aura, which again suits me fine.
Let's not forget that this is a VW Golf, with its TARDIS-like capacity for fitting passengers and cargo, something which the S3 and STI simply can't match in their sedan configurations. And like all other MK7 Golfs with their MQB structure, the car is solid and built with great tactility all around--just like it's Audi cousin, and miles ahead of what Subaru provides.
But my last reason for buying the upcoming R over the GTI is something I'm a bit hesitant to admit: I like the exclusivity of the R. There is something to knowing that VW will be selling tens of thousands of GTI's in the US, but only a handful of R's (500 of the 2015 vintage). Most people on the street won't know nor notice, but I will, and that's important--at least to me. Just like 95% of the time when I drive I won't be pushing the car anywhere near its limits, but all the time which I'll be driving I'll be doing so knowing what the car could do if I chose to do so. And for that 5% of the rest of the time behind the wheel, I guess that's where Car & Driver and I will likely differ. . .
