I went for the forge as I got it for a good price c£450 - didnt fit it myself but it does look quite easy if you have the tools - jack, axle stands etc and a place to do it.
the s3 used to be a good price but lately Vag part prices have shot up and with with the longer fitting time probably another 1hr+ involved you would be a better off with the forge / ths which gives significant gains over the s3 i/c.
also when you come to sell up you can easily whip the forge off and flog it on - with the s3 / ths its pretty much a fit and forget