I've owned a lot of Audis and a lot of BMWs over the years. They both have their pros and cons. Audis have really nice interiors and exteriors, just like the F10's do.
BMW steering is still miles ahead of Audi, but I think Quattro is a better system over X-drive. The S6 is supposed to be slotted between an M5 and a 550i, so it won't beat an M5, but it should be better than a 550 in most cases performance-wise.
Just like an S5 is not an M3, it's also not a 335 either.