Based on having driven the 599 and speaking to people that have driven the DBS, the DB9 is a better GT car, the 599 is a better car.
The 599's precise steering and sensitive throttle make it, to me, feel like more of a sports car than GT; a lot of fun on Saturday afternoon, but not something in which you would want to actually touring the Continent. I've been told the steering and throttle on the DBS are less sensitive and are such that you really get the feeling you could drive it all day.
The 599 is a better car; better built, faster, better handling, more reliable. The 599 also has a fantastic ass, but the DBS is stunning from all angles. The DBS might even win the charisma award.
While it's not particularly impressive in the looks department, I'd still take the 612 Scaglietti as a GT car or a 430 Scuderia over all.
"The Metric System is the tool of the Devil! My car gets 40 Rods to the Hogshead and that's the ways I likes it!" -Grandpa Simpson