I have read stats on these cars and my overall impression would be that theres not alot in it between these two !!!
S3 0-60 = 6.4
0-100 = 16.4
172 0-60 = 6.6
0-100 = 17.9
my info is from http://www.syclone.freeserve.co.uk/rivals_cars.htm
and i wouldnt like to guess which has the best handling !?!