Versus Nikon's 200-400mm VR
I've never understood the 200-400mm VR. Its zoom range is so narrow that it doesn't really count as a zoom, and it's a $7,000 lens.
It's also much bigger and heavier than this lens, and doesn't focus much closer.
The reason to get the $7,000 version is to look important, and if you plan to abuse it physically and environmentally. It will take more abuse than this 200-500mm.
The 200-400mm is a stop faster, which means it works better with teleconverters (use a 1.4x for a 280-560mm f/5.6), but hold on: I'm going to pay $7,000 and buy a 1.4x converter to make essentially the same thing as this 200-500? I'd rather be able to zoom-out to 200mm than 280mm!
I wouldn't want to hand-hold the 200-400, while the 200-500 is intended for hand-holding.
The 200-400mm is a heavy-duty lens for heavy-duty shooters. People love it, but it's a more specialized lens than this 200-500mm.