First of all, the V90 has a sleek and modern design, while the A6 Avant has a more traditional and conservative design. The V90 has a longer and lower profile, while the A6 Avant has a taller and more upright stance. Inside, both cars have luxurious and high-quality interiors, but the V90 has a more spacious and open cabin with more headroom and legroom in the rear seats. The V90 also has a larger cargo area than the A6 Avant. However, the A6 Avant has a more advanced infotainment system than the V90. Overall, both the Volvo V90 and Audi A6 Avant are excellent luxury estate cars that offer similar features and capabilities, but each car has its own unique strengths and weaknesses.