Both the new S6 and RS4 will have 420-450 bhp. The S6 will have a V10 engine and be a little heavier and more practical. The RS4 will be just a little more focused and performance oriented. But it should be pretty close.
I have to admit that the new S6 looks like it'll be a sensational car. i love the new A6 Avant. The driving position and ergonomics are excellent. It also has a very decent amount of loadspace.
Currently in Switzerland and everyone is driving A6 Avants. Quattro in the snow makes perfect sense.
Longitudinally-mounted DSG in the RS4 Avant would clinch it for me. However, I think this is unlikely. My mole in Ingolstadt tells me it doesn't fit.
Will we get longitudinal DSG in the S6. For sure, but not immediately.