Technically, I think the 1.8G has a better lens coating and stuff, which should theoretically give you better image quality, but I doubt for your purposes and level you would need the improved coating but what you would want is the shallow depth of field and the bokeh that is achievable from that fast lens. So since you already own a D90 which has internal built in focusing motor, then you might as well save up the extra cash and get the 1.8D. I dont think you would notice the difference.