If that bothers you then you are only ever going to end up with a car which he likes or approves of & not what you want or like. Personally I think as long as you're happy with the car you own who gives a monkeys what other people think.
Anyway getting back to the first 2 cars you mentioned then...
BMW 535D ~ M Sport version, 2007, 70K miles with the more powerful 282Bhp engine here for £12K: http://www.autotrader.co.uk/classified/advert/201607296290614?search-target=usedcars&make=bmw&model=5_series&price-to=12000&fuel-type=diesel&zero-to-60=fast&searchcontext=default&radius=1500&postcode=cf832nq&page=1&sort=pricedesc&onesearchad=new%2Cnearlynew%2Cused&logcode=p&adPos=2
Audi A6 3.0 TDi Le Mans ~ 2009, 60K miles with Sat Nav etc here for £11990: http://www.autotrader.co.uk/classified/advert/201604193114090?search-target=usedcars&make=audi&model=a6_saloon&price-to=12000&fuel-type=diesel&zero-to-60=fast&searchcontext=default&radius=1500&postcode=cf832nq&page=1&sort=pricedesc&onesearchad=new%2Cnearlynew%2Cused&body-type=saloon&logcode=p&adPos=2
Of those two cars for me it would be a difficult decision as I prefer the exterior look of the BMW and its slightly better performance (0-60 although not much in it) but then I much prefer the interior in the Audi as well as it's 4wd Quattro system which would be useful in bad weather (although not a necessity in this country).
Also the specs list for both cars is very, very good so in short it really would come down to personal preference, possibly any insurance cost differences, driving each to see which one was preferred driving/seating position wise etc or flipping a coin.