Randy, I tried the Sigma 120-300 and it was OK. I also shot the Nikon 200-400 and really liked it and bought a used one (second version). Then GAS struck and the desire for the 180-400 was just too strong. I sold the 200-400 to a friend and bought the 180-400. This is the lens with the 1X extender built-in. I love the lens and use it with a monopod. The focus seems, to me, to be very fast. The VR is a shot saver. The lens is not a light-weight but I rarely shoot hand-held. My friend tried it on a Z7 and was very please with his test shots. I will be trekking around the Everglades this winter in cooler weather so I will see if the size and weight wear me down.
I am quite sure the Nikon 120-300 will be of similar quality and with a D6 on the horizon, yes 2020 will be a new experience. Although, I would have to be BLOWN away by the features of the D6 to even consider selling or trading my D5. Keep shooting, Dale