Most of the time I use this lens: Nikkor 300mm f/4D ED IF AF-S. You can read a review here:
[link]I don't have any experiences with Sigma lenses, except for the 10-20mm wide angle lens. I just checked your gallery and the shots of the Snow Owl are very nice. Just keep shooting and practising with your 120-400mm and you will get more good results. If you prefer to shoot handheld, you should definitely choose the 1.4x converter, since the 2x is very difficult to use without a tripod. I think Kenko's converter is better (more useful), since Nikon's converter is only for manual focus, even it's much more expensive.
Have a nice weekend too