Normally if your wanting a tuning platform id say "go jap" but on this occassion i wont. If its between a tt 225 or a s2k id say take the audi. the s2k is a revvy little so and so with little torque till the revs kick in. the problem with driving it well is you have to be in the vetec rev band at the right time or nothing really happens. i found this with some of my mivec cars (variable cam/valves).
the tuning possibilities for the s2k are limited unless expense is no problem. fliter and exhaust =20 odd bhp on it and thats about it till you do individual throttle boddies, turbo conversion or supercharger. both of those will be over 5k to do.
where as with the tt you already have a better tuning platform: strong and capable engine and intercooled turbo. so for less money you can get better results. also the tt will be a cheaper car to pick up. however its far more common than the honda as its on every street corner. the build quality is much better in the tt too.
but you know if your talking about getting a £10000 car and spending £3000 say modding it. why not get something like an M3 evo or a bmwz3m or an audis4 or tvr chimera 5ltr. much better than a tt!!!
Have you considered a mitsubishi evolution?
ps- they weigh in at 1400kg