i wonder if there is someone out who has experience with both the nikon 200-400/4 and the sigma 120-300/2.8.
i see the advantage that the sigma can be used at 2.8, but to have a longer reach needs to mount the 1.4 converter.
my use is mainly for sports (soccer mainly) and a bit of birding.
The 200-400 has better IQ, and the copy-to-copy variation of the Sigma is greater so you may have to do more work to find or get a sharp copy. The VR on the Nikon is also IMHO better. With a strict focus on performance, quality, and long-term value retention, I'd always choose the 200-400 (and did).
The 200-400 is so good optically that it's often compared to the image quality of the 300/2.8 and the 400/2.8 primes. See this recent thread for a discussion, including some great images as examples. I honestly don't think the Sigma can compete at this level.
Each lens does have a range of focal lengths that the other does not, however, so your preference may be affected by whether 120-199 is more--or less--important than 301-400. Since you mention the concern about additional reach, both can be used with a TC, giving you either a 168-420/4 or a 280-560/5.6. The Nikon provides essentially identical image quality with the TC, and I don't know how the Sigma performs so I can't offer an opinion there.
Finally, the Sigma is way cheaper, which may also affect your choice. If you can't afford the 200-400, then the fact that it's a better lens really doesn't matter.
I have owned the Sigma 120-300mm and used it for birding. The focus lock was a bit slower and never seemed to be able to lock onto a fast moving bird. This is when I was shooting Canon. I shot with the Canon 300mm F2.8 and the focus lock during AI Servo or (continuous) mode locked on very well. The sigma never seemed to be able to keep up with the focus lock. I have the 200-400mm at home but have not used it for birding yet. You will not be happy with the 120-300mm. Me and two of my friends both had it and kept it about a month. All three of us got rid of them. Now if your shooting something slow moving like a person running, (sports) I'm sure its ok. I loved the IQ from the Sigma but it just couldn't keep up. I'm sure someone here with more experience with he 200-400 could let you know about that lens. I haven’t thoroughly tested it yet.
The Sigma wasn't a bad lens for the money when they came out. My first Canon version I think I bought new for $1699, now they sell for over $3000 last time I checked and its just not worth that type of money I feel.
The AF has a bit of a "stutter" effect instead of the near instant "snap" of the Nikon AF-S. Additionally the Sigma really doesn't focus very well while zooming. You'd have to zoom, focus, shoot, zoom, focus, shoot. Its just not parafocal enough to do both at the same time, unlike the 200-400 which can track a player as they come towards you and your continaually adjusting the framing on the fly.
Its not too bad optically if you've got a good copy but there is a lot of variation out there.
Overall its a lens that isn't bad, but that isn't a true contender. You just don't see the 120-300's being used by anyone other than some hobbyist at High School games, who wanted the longest/faster glass they could get their hands on at a more budget friendly price.
I'm not saying that to sound disrespectful at all mind you. Its just that in all my years of shooting I've come across very, very few professionals using a 120-300. Its always 300s, 400's or lately the 200-400
Those Nikon (or Canon) lenses are just in a totally difference league than the Sigma on many levels.
Some Sigma lenses are very good, I love my Sigma fisheye for example, I think its actually better than Nikon's own, but for the most part Sigma's are a second class lenses and the 120-300 falls into that group.
Perhaps something you might be happy with if you can pick one up used for a fair price, but if you've used any Nikon or Canon lenses you'll be disappointed in comparison because the overall performance isn't there.
I own the 120-300 and it is a nice lens for the price. I have shot with the 300 2.8 for a number of years but sold it because of the flexibility of the zoom. I do tend to use it more at the 300 then any where else but in a pinch and when I need to I have the shorter focal lengths. The Sigma IQ does not suffer with the 1.4X, but the focus does suffer. In one shot mode this lens is sharp and and very accurate. In continuous mode I would say I have a 75% keeper rate. The focus can jump a bit. Here are a few examples.
