Nikon 80-400 vs Sigma 50-500 OS

i have heard mixed reviews regarding both lenses. the nikon was the first tele zoom to incorporate vr and also lacks af-s. this makes it a little slow. also, beyond 300mm its IQ drops a little.
the sigma however, may, or may not, have a better, ( but noisier) optical stabiliser. IQ is known to drop beyond 320mm leaving centre sharpness at 500mm rather poor whatever the aperture..
i have considered both of these lenses and decided to go for the nikon 300mm f4 plus a 1.4 tc. although it has no optical stabiliser image quality is far superior. i would rather this fixed focal length with a good reputation rather than take a chance with either of the other two.

Can't comment on the Sigma - never used it. I do own and use the AF-Nikkor 80-400 on my D300. Whether or not you will be happy with it depends on what you are shooting and under what lighting conditions. AF on that lens is slow - but once you put a TC on the 300/4 AF-S, the AF speed there drops down to about the same level. There should be plenty of 80-400 lenses on the used market - I would not pay for a new one. My suggestion would be to get your hands on a copy of each and then decide.

I have the Sigma 120-400 HSM OS lens on my D300s and find it to be very good. Here are a couple of quick shots. These were in a room with limited light and using a tripod.

Hers is one at 400 mm.

This is the Space Shuttle I shot Friday from the roof of The Grove Los Angeles parking lot. Used a polarizing filter that did not work too well. Cropped and color corrected.

I have a photography friend who uses the Sigma 50 - 500mm. She's had many issues with it. Finally she found out from someone at Sigma that the lens is very sensitive to motion etc. She takes extreme care of it. Far beyond anything I'd consider normal for a lens. All because the elements are so susceptible to issues. I'm trying to remember her words.
Anyhow, the 120-400 is supposed to be a lot better. It's a newer lens. If I was you, I'd either go with the Nikon or the newer 120-400 - - super zooms like the 50-500 are asked to do a lot which they can't always live up to.

Correction, the shuttle shot was not cropped.

The realities of mass production lead to an unavoidable fact - all of the finished copies of a lens coming off a production line are not equal. There are good copies and bad copies, all assembled from individual parts each of which meets its specifications. The more a manufacturer tries to reduce the price or to include more features for a given price, the more pressure there is to accept components at looser acceptance criteria, which leads to wider variation in finished products.

So I see one person post that his new Sigma 50-500 gives him fabulous sharpness and contrast, while another posts his new 50-500 gives him nothing but junk. Both are probably right - that's the reality of good and bad copies.

I said all of this to lead to a decision of mine. Because of the problem of "bad" copies of less expensive third party lenses, I decided I would only buy a longer lens (in the range you are considering) after I had put the lens on my camera body, taken a few shots with it, and found those shots to be acceptable. And I would only buy the one on my camera - I would not take another in a box.

So I took my 80-400 I've had for 8 years and went to a local store. They had no 50-500, but they had a Sigma 150-500. I set up my tripod outside the camera store's front door and took a couple of 80-400 shots using my ML-L3 remote. Then I put on the Sigma 150-500, focused on the same subject and took a couple of shots in the same way. The Sigma images were obviously inferior to my 80-400, no doubt about it, and I didn't buy the Sigma. I've seen photos taken with a Sigma 150-500 and posted to various forums showing excellent sharpness, far better than I saw from the copy I tried. That's why I wanted to try the lens before buying.

An alternative: Tamron makes a 200-500, but it has no VR or OS built in. I shoot everything on a tripod, so that wasn't important to me, and my direct comparison test found it to give me images equal to or better than my 80-400. I bought it.

However, now I find I use it less than the 80-400. I'm very much used to the 80-400 zoom range, and I use all of it. I also keep a 16-85 on a second camera body, so I have a range of 16-400 covered with no gaps. When I have the 200-500 mounted, I find myself with a gap between 85 and 200 that has proven to be a problem at times. You didn't say what other lenses you have and only talked about the longer end of the ranges of the Nikon and Sigma lenses, but the zoom range is something to consider in choosing. Based on reputation, I think you'll get the best image quality from a Nikon 300/4 with Nikon 1.4x TC, but that only offers two focal lengths, 300 and 420, no zoom, and nothing shorter than 300. That would leave an undesirable gap for me. YMMV.

I also agree with Dieter - the 80-400 has been around for a long time and there are lots of good used ones available. I would not pay $1600+ for a new one when I can get one graded as Excellent at KEH for $1000. And, if the 200-500 range meets your needs and VR isn't necessary, the Tamron can be had for under $1000 new.

I haven't heard of reliability issues with Sigma, but i haven't owned one. However, based on the production variation issue, I strongly recommend that you accept the idea of paying a bit more for the lens in a brick-and-mortar store compared to purchasing online, go to local stores and try lenses on your camera, and only buy after you've verified yourself that the copy you are buying gives you the image quality you expect.

If you are looking for a tele lens in the Nikon's line-up which is also affordable, then you should go with 300mm f/4 af-s + tc-14e as a first choice. Then, my suggestion would be sigma 50-500. Skip the 120-400 and the 150-500 as they are not EX series. Always look for the gold ring in front of the barrel, this means EX series in Sigma which means superior optics. I have seen many sharp and beatiful images taken by 50-500mm. You can also find the reviews of 120-400mm and 150-500mm in Thom Hogan's web site where he thinks that these lenses offer poor acuity.
Nikon should launch a 400mm f/5.6 af-s vr that would turn many wildlife photographers into Nikon from Canon. Currently, many amatures and new starters buy Canon equipment just for the reason of this lens, 400mm f/5.6. Similarly, 800mm vr lens was also necessary in Nikon's line-up, they finally announced it. In my own wish list though, I would much rather seeing a 500mm f/5.6 af-s vr and/or a 600mm f/5.6 af-s vr which would be much more affordable than the current 500 and 600mm f/4 lenses, and offer beatiful focal lengths. However, I highly doubt that we will see such lenses.
By the way, the images shown in this post taken by various lenses do not reflect any conclusion or show anythink about these lenses' performances. One should take pictures of distant objects and crop 100% at least and show the results without processing, and if possible the shots should be taken in clear weather consecutively with different lenses, then the results start to convey some meaning. Lens testing is a difficult and care taken procedure.