A:Answer The 799 lens is for full frame ((FX) cameras, the 399 version is for crop sensor (DX) cameras. The 399 lens also has a plastic camera mount which may not be as sturdy as the metal mount on the more expensive lens. the "P" lenses are both newer versions.
The lower priced lens may not be to the optical standards of the more expensive lens, but it is still very good.
A lens hood is a necessity to decrease flare and hazing from off center light and that is an extra.
All in all, the more expensive lens is a bargain but if you don't expect to ever go to the full frame cameras, or make prints over 13x19 inches, you will be happy with saving the money.