This is a good point. As much as @wev uses the long lens, I would definitely recommend going with the 200-500mm if he could get enough for the Tammy without losing too much. I'm not trying to be a fanboy of the Nikon 200-500mm lens, but it is truly an awesome lens. I use to love shooting flowers and butterflies with my 70-300mm but this really blows it out of the water focus and IQ wise. Not to mention bird and BIF shots.
I don't know about the Sigma so I can't comment on it.
My money would go on the 200-500,hell even you get great results