Without driving either car i coulddnt say for definate but i do hold the opinion of EVO magazine highly. They rate the standard GTI just over the S3 due to the clinical way in that it does its business.
VWGolf GTI (Mk5) (3dr)+ Character and ability: the original GTI is back - But at a price
Audi S3 + Very fast, very effective. verry, err quality...- A little too clinical
Now how much 'joe bloggs' would recognise the ability of the S3 as negative is questionable. IMO i would say that if you wanted fast, sure footed cross country pase then the S3 would probably be the better choice. If however, you prefered to realy drive (on a private road of course) then the adjustability of the GTI on the limit would make it the more engaging car to drive. At this price range power and speed should not come into it, remember 2nd hand Porsche's are available from £15k.
Either way, both cars would give any owner emense satisfaction!! Looks and how you like you car to 'feel' will be the deciding factors.
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Oh and i would have the GTI / ED30 allllll day long
