In my very scientific testing tonight, Forza 5 on the Xbox One, I have come up with some interesting differences between the R and the S4. Unfortunately the game only has the Mk VI R so I could not get a true test.
The A pillar on the Golf, being more vertical and thinner makes for a dramatically better view of the track than the S4. I am pretty sure a Mini Cooper could hide in the visibility block from the A pillar on the S4. The larger side mirrors on the S4 were much more usable than the smallish ones on the R.
The R seems to be much easier to coax through the tight curves of the Nuburgring, and is more stable when using the curbs. The R is more stable and does not lumber through corners, the rear end tends to not throw off the balance and try to come around on you nearly as much as the S4. The S4 seems to be good with early power in the curves, but brakes seem to unsettle it unnaturally. Top speed on the finish line straight was dramatically different, 165 for the S4 and 131 for the R. Not sure how accurate that is, or if the Mk VII R might be a bit faster, but that long straight was a big part of the time difference between the two, as the R is quicker through the crazy windy areas.
Best clean Ring lap on the Golf R 12:07 and the best time on the S4 11:32. Might have to go test drive a few more options tonight.