My son has had both and he prefers the S3/TTS engine and the fact the engine seems to pull better from lower revs. The DSG gearbox will be less of a headache if you have the car remapped. The newer cars respond better to a remap with 360bhp being available with just a stage 1.
You are having a new car but the early 7R suffered from turbo problems in case you decide to save some cash.
S3 gives you genuine sound whereas the 7 R has piped in sound.
Lots more to play with on the 7R regarding settings.
Some people, myself included prefer the driver involvement with a manual. With a DSG you just sit there and push the accelerator unless you fancy manual mode. So everyone can have a go at Santa Pod with a DSG.
The manual 7R can get through a clutch in 7000 miles as my son found out. Not good news and he did not use the car for drag racing.