I would say go with the S7. I test drove an A7 and its just not very impressive performance wise to be frank. The S7 with the little v6 with "a little more power" has the same power numbers as the "monster v8" from the pervious generation. My MPG is 22mpg with the 2.9L with an APR tune. The 2.9 has been around in Porsche cars for years with no real serious issues. I have 55k miles on mine and the only issue I had as the alternator recall and a water pump at 40k miles.

DSG transmissions are not as strong as the ZF 8sp that's in the S7 and been in the performance cars for a long time.

Not sure if you can get the sport differential in an A7 but the 4 wheel steering is a huge plus for me in tight city driving.

When I was buying mine I was looking at the c8 S7 compared to the c7 S7 having come from a heavily modded A6 with the 3.0T and was concerned about the potential of the 2.9L and Ive seen some crazy Porsche builds with the 2.9L so I went that route and the new interior.