Unless you are doing 18,000 miles a year can I suggest you look at total cost of ownership and not fuel.
You will lose far more in depreciation then you would ever save in petrol costs, unless of course you are doing massive high mileage or buy a second hand motor.
If you are buying new you may want to get the S3 simply as a result of class leading residuals, my current 8L is approx £10K after purchase new of £22K over 5 1/2 years, along with variable servicing which meant a bill of around £1500 for approx 64000 miles and that just left tyres and oil.
I expect the new S3 will deliver the same levels of TCO as the old one and offer improved dynamics when driven too. Great!