I'm surprised that they are using -300s as the extra weight of the longer upper deck (as compared to the -200 & -400F) means less payload can be carried, unless the freight is bulky in which case the upper deck provides more space.
The -300 upper deck provides no extra space over a -200, as there's no way to get taller pallets into that area.
They're slightly heavier, though slightly more fuel efficient and 0.01 mach faster, that's all.