Tue 08-Jan-13 12:40 AM | edited Tue 08-Jan-13 01:00 AM by NatureDon
As already stated, both are excellent lenses and also very different. I'll offer my 2 cents since I owned the 200-400 for about 3 years, then sold it and bought the 300f2.8 which I have used for the last 3 years in combination with TC1.4 and TC1.7. For both lenses, my primary target was winter song birds.
First, I would not sell my 300f2.8 and buy a 200-400 just to get "more reach". Both of the TCs I use on the 300 work great with that lens. Just looking at the "more reach" factor, IMO the 300 f/2.8 + 1.7 is better than the 200-400 + 1.4 at producing sharp feather detail.
On the other hand, comparing the 300 + 1.4 wide open at f/4 to the 200-400 wide open at 400mm and f/4, I think the zoom lens was better (not overall at 400mm for fine feather detail, just wide open...IMO).
If you want a zoom, then that is where the 200-400 shines. If you want the best 300mm you can possibly get, then the 300mm wins.
To sum up, I would not sell either lens to buy the other unless I specifically wanted a zoom or I specifically wanted the best 300 lens out there. They are too close otherwise and both are extremely good. I have no intention of selling my 300mm, however I wish I had never sold my 200-400 .
If you want more reach than you already have, I think you need to look at the 400 f2.8, 500 f4, or 600 f4 in the Nikon line, or look at third party options.
YMMV of course, and I would bet my opinions stated above are arguable depending of exact experiences and uses each person puts one lens or the other through.
Edit to add...after looking at your post again, you mention handholding. in my picture at left, I am handholding the 200-400 although leaning against my covered boat for a little extra steadiness as I photographed a sparrow. My wife saw me out there in frigid weather, thought I was nuts, and took the photo from the house. I liked handholding the zoom a little better just because of the way it fit my hands.