how much does a DSG affect the fuel economy 
The point of DSG is that it's not a conventional auto - the economy with DSG *should* be better than a manual.
I'd get a diesel, because even if you spank it you won't get spanked on the fuel bills. When we test drove a G6 GTI it did 14MPG. No matter how hard I spank our 4Mo Passat it won't return lower than about 28MPG. If you get even remotely frisky with the throttle you are punished hard in a petrol. Diesel you're not.
Add to that modern petrol engines rev like diesels and modern diesels rev much harder than diesels used to and you're in a world gone so mad that we won't spank the monkey, the monkey will spank us.