Well if you look at the numbers, your A3 has 247hp and weighs 1495kgs, giving 167hp/ton
The S4 has 340hp and weighs 1700kgs giving 203hp/ton
So in a straight line the S4 should be significantly quicker (Theoretical 13.8sec quartermile vs 14.6 for the A3)
As for handling, well the S4 is a heavier car which will detract from it a little, however it does have a better suspension design as far as i know (A3 uses mac struts?) which will help the S4 a bit. I'd say that unless your pushing it to the very limits of grip they should be pretty similar, although the longitudinal design of the S4 means it will feel a little more nose heavy compared with the A3
All round the S4 should be a better package. Might be worth getting a test drive to see if its right for you though...
