I was in a similar situation, as I've had pretty much every type of engine going apart from a V8 so was sorely tempted by an earlier S5. However the fuel economy for me around town was a bitter pill to swallow as around 50 % of the time I'm nose to bumper in the daily grind. Plus the Bitdi does have the exhaust resonator that gives it a deep grumble, which worked for me.
Now I know that you are probably looking at the v6 supercharged version, which by all accounts is a stormer of an engine, and highly tuneable, but it wasn't enough to convince me out of the A6.
The other thing to factor in is that the A5 / S5 / Q5 interior is showing its age (a bit) so the A6 being a generation ahead swung it for me.
I needed a 4 door too, for the family, so options were sort of limited.....
That said however, my next motor will most likely be the S4 avant when it finally hits the market, and to hell with the fuel consumption.....!
(Although that F pace is really really starting to grow on me)