I have the 100-500 and, as a wildlife and nature photographer, it serves me well. The 300-600 would most likely not focus as close, but that remains to be seen. It will miss everything from 100-300. A week ago, I was searching my photo library to see where I shot most of my images with the 100-500. A friend was debating to get the lens or a fixed focal length. About 30% of my images were shot between 100 and 200. About 20% between 200 and 400 the remaining 50% were between 400 and 500. I do mainly mammals, reptiles, amphibians, wildflowers, insects (butterflies, dragonflies and spiders (I know spiders are not insects). I do some bird photography but like raptors more than tiny kinglets and warblers. I do enjoy woodpeckers. The 100-500 has met my needs for the variety of wildlife I enjoy capturing. Once in a while, I would like more focal length (think waterfowl way out in the wetlands) but, honestly, atmospheric conditions tend to ruin long shots anyway. The 2/3 stop from 5.6 to 7.1 really isn't that significant in real world nature settings, at least to me. Others may disagree and that is fine, I'm talking about my photography.
I'm sure the 300-600 would sell but it holds little interest to me over the 100-500.