I was in a similar boat last year. I had a 99 c5 convertible with a t56 and a fiat for the city. I was originally looking at a s3, but a used r came up for sell here and i ended up getting it. The r i bought ending up have a apr tune on it, so my r is faster then stock. I have not driven hard, pedal all the way down, a stock r or a s3.
Here is way i can tell you coming from a c5. The Visceral feel of the vette is not what you will get out of the r. The r feels almost german lux/hot hatch to me. The difference with the gti, my parents had a mk6 gti, and the difference with the MQB chassis is much nicer. The chassis on the mk7 is just brilliant in my opinion.
What you will get with the r, is a car that is much more confidence inspiring then the c5. My vette gave me pucker factors much easier and at a lower speeds. You can be a good bit more lax on proper driving techniques in the r, but it also handles better then my c5. If you get DCC, its completely comfy when you want, and a little monster when you want. With a tune acceleration is just stupid. Its pretty silly actually.
I would ask yourself what it is you want. If you like the visceral experience of a analog sports car, the r won't be for you. If thats is not a big deal, the r won't disappoint. The golf r is faster, more practical and comfortable then it has any right being. I do feel that it has fill both rolls with me as a comfy city car and back roading monster.
I would also keep in mind with the decision making process. They don't make steering wheel positions sensors or pcm computer modules for that year c5 any more.