I would say the Audi is the better engine, ( I can only really comment on diesel because this is what I work on ) merc are also reliable but very expensive when they go wrong, merc make engines for merc where as Audi also includes vw, seat, Skoda so this tends to bring the price of parts down, as for the handling you won’t get better than Audi for 4 wheel drive, I took the wife’s car out a couple of weeks ago and had a bit of a play with an M4, the BMW is a much more powerful car but I left him for dead as he couldn’t cope with corners