I just got back from test driving a brand new rs3 (had about 50miles on the clock). Having come from my s3 that I've had for nearly a year from brand new, I was underwhelmed. The looks are identical apart from a few tiny details, most notably the front grill and stance - it is a lot more aggressive. I found the acceleration a bit laggy and less responsive than my s3. The steering also felt lighter in dynamic than my car, not sure I liked it tbh (although many of you may disagree with me on that)
The noise was absolutely epic. Exhaust sounds incredible, start up, revving, acceleration etc. awesome. However, the farts between gears are less prominent on the rs3 than the s3. Mainly because there's so much more noise it probably masks it a little. I also found the gear changes a little jerkier than the s3, and again, felt a little disengaged from the car in terms of driving "feel".
I didn't even bother doing the figures on it, I already made my mind up that my s3 was good enough and I can't justify going from mine to a new rs3. Would cost me over 10k when all is said and done.
However, when I got back into my s3 I realised how good it was! I think I felt so underwhelmed through the test drive I didn't take in all the aspects to enjoy them - I criticised them instead. But stepping down from the rs3 into the s3 felt like a much bigger jump than it did moving up.
My advice is, if you have/or going to buy an s3, stick with it and don't even test drive the rs3. If you can afford it however, it's every bit as good as the s3 and more. I reckon if I had it for a couple of days I would have totally and utterly fallen in love. It took for me to get back into mine to realise how good it was.
My two pence worth anyway....