My understanding is you get the same image with the 50mm DX and the 50mm FX on the DX body, but the DX lens is usually cheaper and more compact.
D means there is an aperture ring, yes. But D lenses also don’t have an internal focus motor so if the D70 body doesn’t have it’s own focus motor it’ll work on your camera but at the same time it won’t. Like I can technically use D lenses on my D3100 but only at the narrowest aperture and it doesn’t AF so it’s not worth the savings at all. The mount on the G lens is better though and it’s heavier because there’s less plastic in it. I have the 18-55 kit lens and also a 28-85mm D lens and there is a pretty significant difference in quality between the two mounts and lens elements.