The G lens, being an AF-S lens, will focus a bit more quietly. It will autofocus on bodies without an AF motor, which is of no benefit to a D7000 owner.

Both are sharp. The G lens is a bit sharper wide open. It's bigger, less plasticy and looks nicer than the D. I think you'll be hard pressed to tell the difference in sharpness between the two in general use.

The G lens has rounded blades, so might give better bokeh, but this is subjective.

It also has a manual focus override, so you can manually focus without switching the lens to M.

I should mention that the "G" designation means that the lens does not have an aperture ring, which prevents it from working well with a film camera. Not needed with the D7000.

I've upgraded from the 1.8D to the 1.4G myself. Would have been happy with the 1.8G but found a good second-hand 1.4G. Obviously I thought it worth upgrading, but the differences are incremental and subjective. I think the manual focus override and slight improvement in sharpness and bokeh worth it for me, but it's really an personal choice. If you can visit a shop and try them on your body, you may want to buy the G lens just on looks and ergonomics alone.