If you are having it from a dealer I would arrange an extended test drive, the least they can do. I have driven the SQ5 and A7 bi-TDI and they are a good step up in performance over the standard 3.0 TDI. Along with the exhaust noise trickery they use, especially in the SQ5 which sounds like a v8 , I think they are a good choice.
They use a speaker in each side of the exhaust system. Kind of an extra muffler each side that feeds into the exhaust. You can get an add on module that intensifies the effect as well.
It may now be a V8 but it has more torque than most V8's.
Another one here with one, utterly lovely car.
The Torque is endless and works well with the box, and for a big car it's pretty good on a back road blast.
Even without the sound generator in the exhaust it does sound good, but as others have said its a laugh with it on, had a couple of people comment about they thought it was a V8 hahah, overrun in Dynamic mode can generate "exhaust cackle", it may be false but it's no worse than sticking an aftermarket exhaust on for extra noise just a techy way of doing it instead.
I've yet to see any real maintenance costs but talking with the local dealer before hand it's the same as all the other 3.0ltrs, even the monster brakes aren't that pricey to change, depending on the spec you get (recommend a BE ;-) ) the 20" wheels can be an "ouch" moment when looking at tyres but shopping around gets the price down.
You do need a good run to get the fuel economy, round town does expect much more than 20-25.
S6 is different proposition. The rarity of the car is helping residuals, but they don't sell quickly as it has higher running costs, but either BiTDI or S6 is good investment. If you don't need the diesel, the extra spec for the S6 is worth the price difference. Better seats, suspension, brakes all add up to make like for like very similar price to BiTDI. Whatever you get, spec matters for resale.
I am thinking of trading my A6 BiTDI for an S8 if you're interested