Punchy Dan:
0just purchased tfsi 2.0 top spec S3 auto Quattro 64 reg
I got rid of my S3 few years back. White in colour and black edition model. Not everybodys cup of tea but looked smart imo and brilliant to drive. I was surprised to learn an audi ( cant mind exact model or year ) 1.8 turbo was actually quicker than S3 0 to 60 ( not by much but still quicker ) due to S3 carrying more weight than the 1.8 turbo. Its the same when you compare the S4 to the RS4. RS4 more expensive and your getting an older model compared to what you would get if you opted for S4 and again, hardly any difference in performance of both. You probably wouldnt notice difference unless you were trying really hard to notice. I looked at both for a bit and considered buying one or the other but opted to be sensible and buy a more economical Audi. Need to save the pennies for a bit before i tell the missus am going shopping and coming home with a Nissan GTR 
. Il be wearing body armour and a crash helmet when i tell her just incase, on the off chance she might not be overly delighted 