Id get an R, if your buying the Mk6, you can get a pretty decent price on them at the minute and I think you would just be saying to yourself after 6 month why didn't I.
Granted the fuel will be worse but not that much worse, your talking maybe 30-40 miles less per tank, that's not a deal breaker in my book. Plus you NEVER see them, I love that in a car I see a Mk6 R maybe once every few months and I see a Mk6 GTI about once every few weeks so that gives me the feeling of something abit more exclusive.
As for the Mk 7 R Jesus I see about 3 a day they are shifting those things which is good for me cos the second hand market will be flooded

S3's are nice but as above much more common, well in Leeds anyway. Plus I just prefer the R design so, just my opinion...