I think the Audi would be more spacious than the VW.However, wouldn't you need to change out the entire fuel system, like the fuel tank, lines, etc., to run diesel? What engine would you use then? The Audi 5000 diesel (yes, there was one in the early 1980s), was quite a dog performance wise. And I don't think it had the quattro driveline if you're thinking of AWD. Quattro adds quite a bit of weight and performance-sucking drag on the engine output. That's why all the quattros had turbos or lots of cylinders.