Colleagues at work have both of these cars, I looked long and hard at 200 era Volvo and Audis whilst buying my previous car, and I remember that:
Both the Volvo and the Audi have excellent seats, providing good comfort and support on long journeys. No real difference here.
If you are 6ft or taller, the Volvo V70 had less headroom for the driver - this was my main reason for rejecting it. The saloon S60 and S80 - which are a newer generation - are much better for front headroom and on a par with the Audi.
The 2000 era Volvo V70 D5 used the Audi A6 2.5 TDI engine under licence, but could be had in either 140 bhp or 160+ (better) bhp tune. No real difference here, should get similar or identical towing performance between the makes. Just try to get the higher output engine - it burns less fuel too.
Some time after 2000 Volvo introduced its own D5 Diesel engine and this is OK too.
The self levelling rear suspension is worth looking out for.
In the end I went the German route - because of the V70 headroom - but both are good quality, robust cars.
Robert