The Dex-VI is synthetic like the ATF+4, that's why it's so pricey. It's also very good fluid.
Dexron VI vs Dexron III
The following chart represents the improvements over Dexron III.
Extended Fill Range = 100,000 miles Normal Use.
Change Interval = 50,000 miles Severe Use.
Clutch Friction Stability = Improved 100%
Clutch Durability = Improved 120%
Oil Film = Increased 20%
Fluid Oxidation = Improved 100%
Shear Stability = Improved 200%
Foam / Aeration = Improved 150%