I debated between which long zoom to get between the sigmas and the Nikkor, I ended up buying a nice clean copy of the Nikkor 80-400 VR,, and have been very pleased with the results, I put the Kirk tripod color on which makes it very stable, and makes an excellent hand grip. I have been using the lens 95% as a hand carry, and it really works with the VR on.
Then, I heard thru here how great the 300 F4 was, so.... bought one of those used too,, and yes, it does edge the sharpness of the 80-400 some,, but you loose the flexibility. I still need to buy the TC, but darn its pricy and the used value stays high, so,, most likley going to get that new.
I was very impressed with the build quality of both these lens (especially once replaced the Nikkor Collar on the 80-400), both lens are built like tanks,, which I cannot say the same for
the sigma lens I own (they seem to develop squeeks over time and focus hunting).