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I would agree completely on the comparison to the 200-400, but I'm waffling on the 300 f/2.8 IS II and 1.4x vs 400 DO. The 300 is a pound heavier, but still hand-holdable, has better IS, and the option to be a 300 f2/.8 or 400 f/4, and the price difference isn't so great.
Don't get me wrong, I have a 400 DO in my hands this weekend and it definitely is a bit easier to handhold than the 300. I really am undecided on that comparison.
I recently purchased a used 400 DO and also have the 300 2.8 IS II. The 300 nearly always has a 1.4x or 2.0x on it. I've lugged both around for hours at a stretch (usually no backpack, just shoulder/neck/arm carry). The 400 is lighter, but my lower back seems equally stiff after 2-3 hours of steady carry. I like the 400 DO, but I love the 300 2.8 IS II, even with a 1.4x TC attached to it. If IS is important to you, the 300 is light years ahead. However, the 400 DO still gives you lots of quality for your buck if you purchase a used one. I guess that makes it a cost/benefit analysis for each individual.