This can be as easy a question to answer, as it can be a difficult one to answer.
If all you want is length... then the 80-400mm wins hands down. Especially at it's lower price. They are both VR series, however the difference comes in other areas other than focal length which makes the 70-200mm a more valuable lens, for a price of course.
I have seen some excellent photos with the 80-400mm, as well as the 70-200mm. The Bokeh of the 70-200mm is better than that of the 80-400mm in my opinion. Like night and day. So if Bokeh is your thang, the question is, am I willing to spend the extra money for better Bokeh?
f2.8 of the 70-200mm, lemme type that again, f2.8. Another question for yourself, will a stop or two "faster" be worth the extra money for the 70-200mm when compared to the 80-400mm?
Image quality, sharpness, etc. is better in my opinion with the 70-200mm, how much better, well that is personal opinion. So now you ask yourself is overall improved image quality and sharpness worth paying more money for this lens over the 80-400mm?
The reach, 400mm hard to get to with a 70-200mm, so we price out teleconverters, the 2x Nikon tele is in the price range of 435.00+shipping.
So the cost of the 70-200mm + the 2x tele is $1904.95 (including $100 rebate.) Making your 70-200mm VR a 140-400mm, you can figure out the crop factor! However your f2.8 is now increased by 2 stops and is now an f5.6 all the way across, glass quality is still excellent, however you are adding more glass with the tele. Still excellent Bokeh!. Still very sharp!
The cost of the 80-400mm VR is 1,329.95 (including $100 rebate) at f4.5-f5.6.
No matter what... there will be those times when you say, darn I wish I had an extra 200mm, and there will be times when you also wished you had the ability to drop down a couple of stops.
So... The real question is...
How much money do you have, and what are your needs
If it is any consolation, I bought the 70-200mmVR and the TC1.7 (1.7X) tele. Do I wish I had the 400mm, well actually no, I do wish that I had a 600mm as sometimes those birds will not cooperate and come down from the trees. Do I regret getting the 70-200mm VR? H$%^ No
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