Well i can tell you 1st hand,as i have BOTH!!!!!!!!
The John Cooper Works is great fun,but as said above the S3 is a hell of alot better to be in.... As for performance (which i figure is what your after),the S3 (mines remaped) can beat it in all departments really.. performance,MPG,power,but handling il have to give to the mini,unless its wet,then with out the traction control on,just wheel spins EVERYWHERE!!!!!!
Tbh,its actualy forsale..... If your interest,its MINT!!!!! And ive worked in the motor trade (bodyshops) for 15years,so i know when mint MEANS mint.... 05 plate,genuine JCW,metalic blue,27k,FBMWSH,10 mot,10 tax,3years warrente,4 brand new tyres,TLC,half leathers,HPI clear,and all the trimmings really... If interested PM me,and i can tell you more,and send you pics.......
Pete...