Sigma 300mm 2.8 vs Sigma 120-300 2.8 vs Nikon 300mm f/4 AFS

Hello, I have read several reviews of these lenses, but each seems to come to oposite conclusions.

I know that Nikkor is much cheaper, but I want to use any of these at 300mm with a TC14 to reach 420mm and wide open, so the 2.8 aperture could be very interesting, and the Nikkor 300mm 2.8 VR it's too expensive ...

I put the zoom only because some people say that it is better optically at 300mm and 2.8 than the prime, but I'm not so sure, and the focus speed seems to be slower ...

Any experience with these three lenses? IQ? autofocusing speed? MFD limitations?

Thanks in advance.