S6 maybe better if...
Maybe the S6 is a better choice if you're looking near $80K and need a 4X drive performance car already tuned (engine, exhaust, tires, and suspension) from the factory.
The closest BMW match would be a 550xi with Dynamic Handling Package (ARS + DDC) and M Sport package. You'll save about $5000 but, to really match the S6, you'll want to at least upgrade the 550i exhaust, ditch the RFTs, and lower the suspension on the BMW. Perhaps even the Dinan Stage 3 route which then puts you way ahead performance wise but will eat up the savings you might have realized with the BMW and put you closer to $80K.