I don't think there's any room for dithering there. You'd just end up losing your tiny details.
I think I prefer the lighter colouring, but I think the shadow colour should be a little darker. It contrasts too little with the light side, and too much with the door outline.
I'm confused by the door. Why does it rise above the body of the tank? Why does the outline continue on above, as well? Is the outline a physical object?