Sigma 50-500mm vs Nikkor 80-400mm?

I was wondering which of these two lenses is better? I'm trying to decide between the Nikkor 80-400mm lens or the Sigma 50-500mm lens. I need a lot of zoom so either will work for me. What i'm trying to figure out though is which is better in terms of picture quality, handling, durability, etc? Its for sports/wildlife photography.

The Sigma 50-500mm and Sigma 150-500 lenses have a lot of really good reviews…

It's tempting to use these wide range zooms as a 'walk around' lens, but all lenses have compromises in their design, these wide range zooms have the most compromises of all lenses. The result is image quality suffers.

Things that you need from a lens to get high quality images such as high colour and luminance (B&W) contrast, no distortion, flat field (where all the picture is in focus right across the image), no vignetting (where the image goes darker towards the corners) won't be present in these 8:1 and 10:1 zooms, their performance will really disappoint in all these parameters.

One of the other compromises is that in order to get anything like a decent image they sacrifice aperture. This makes these lenses 'bright day' lenses, your camera will just 'hunt' trying to focus in dim light conditions making them virtually unusable in these conditions.

For image quality a 'prime' lens (just a single focal length) is best and they usually have wide aperture making them ideal for low light shots, used wide open they have a narrow depth of field which you need for 'selective focus' shots (where the subject is sharp and the background blurred). Few compromises in these lenses, image quality is as good as it gets.

Short range zooms (around the 3:1 range) are somewhere between the two, some compromises, but still very usable.

The ideal lenses for sports/wildlife are the virtually universal 70 - 200mm f2.8 (the Sigma EX version is very good), and for the longer end a 100 - 300mm f4 or a 300 - 500 f3.5, any of these lenses will cost more than your camera, but they are definitely what you will aspire to down the line.