I assume you've looked at the price difference and fuel consumption differance. Both are great cars. The 3 litre diesel is quick and powerful compared to most tow cars but in its engine size it's not exactly outstanding for a 3 litre. The 4.2 offers high performance from the awesome V8, the A8 is a class act. Both will tow most caravans with ease and offer 150mph performance. We have 4.2 Q7's, if you want a performance car and very easy towing it has to be the V8. If you want a good quickish car and good towing experience the V6 should not disappoint and the A8 has lots of room and a big boot.
There's a newer Audi 3 litre diesel engine out now or some time soon, more economimical so check out if the A8 has or is expected too have the new engine if buying new.
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