I am not a Nikon man or have any knowledge of the Sigma lenses but in general terms its going to depend on your priorities. If its birding then the 50 -500 is the way to go as its longer and still leaves the option of adding a converter to that for even longer reach when funds permit sometime in the future. I don't know enough about air shows to comment but would imagine a 100-300 zoom with converter would be versatile and long enough for most shots (?) For general use the 50-500 would be way too heavy for me to lug around on the off chance I would need it and the necessary monopod or tripod. The Nikon prime I assume is the best lens optically but loses out heavily on the versatillity side for general use and I suspect at airshows. I use a Kenko Pro 1.4 converter with my Canon lenses and the optical quality is pretty good so I don't see any problems there apart from the loss of one stop. So, birding 50-500, any other usage 100-300 zoom with 1.4 converter.
Hope my thoughts help and happy hunting for whatever you decide. Mike
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