I have the same question but mine a little different because I wonder if it makes a difference choosing between D or G version of any of the 1.4 or 1.8 50mm lens when using the lens reversed and coupled with a telephoto lens for macro photography.
I know G lenses doesn't have aperture ring so wonder if for my purpose the D version are better. I read at kenrockwell.com that the 50mm f1.4G was faster in AF but had worst distortion than 1.4D.
For my intented use of the 50mm makes sense to spend more in the 1.4 version instead of 1.8? there are 300 dlls difference between the 50mm f/1.8D vs the 50mm f/1.4G.
I don't mind in the AF speed but more in the sharpness and distortion and also in the speed when mounted directly in the camera.
P.S. I currently own a D90 body but planning to get the D600.