I forgot about another alternative.
Since your dad is looking at the M6 and the DB9, he might want to consider the Audi RS9. It hasn't come out yet, but it will in a few years.
The RS9 will be considerably faster than either car and since it's a mid engine all-wheel-drive car, it will handle a whole lot better than either car. It has futuristic looks and the price is roughly the same. I think your dad should seriously consider this car.