Buy on condition.

Cars that look similar in pictures and sound similar in descriptions can be very different when you look at them in person, so I would always recommend going and having a look. Having a low mileage can sound like a good thing, but often it means the car has sat for long periods of time or done lots of short journey when the engine is cold. Equally, high mileage cars will have worn suspension, steering, tyres, brakes etc.

I would personally go for the BMW because it's nicer to drive and cheaper to run, however the Audi has a nicer interior with more space.