My average in the 123d is always above 40 mpg, 47 at mo, you can't really compare the 2 tbh, the engines are totally different as are the chassis. A 120d would be a far better car than the a3 if the engine out of Warrenty and mpg concerns you.
I love my 123d as I see it for what it is, it's by far the best 4 pot Diesel engine that feels very much like a petrol, but you can't get away from the fact it's all out of puff at 5k. The mpg is very good considering its 200bhp stock, cheap tax and a good rwd chassis. Just don't think it's going to be as much fun as a focus rs and its a difficult car to beat.