For speed there is no contest, the S3 wins hands down. - but where can you drive at 155mph excluding motorways ? And if you drive at 155mph on a motorway your likely to get your balls felt by the fuzz. Ok, the S3 is limited in its 155mph, but the mk6 GTI aint no slouch either and at a conservative 149mph top speed, its hardly that much slower. S3 is up with an STi in standing start speed but suffers turbo lag I hear too. The GTI is as smooth as the fonz.
S3 is much more thirstier, more expensive to insure, and for each standard car its almost £6k more to buy, running costs are higher and supposedly is very uninspiring to drive.
The GTI all the way, its a no brainer. Although I'm gonna test drive the S3 next weekend just for fun.