I have the 300mm f4, the 400mm DO I, and the 600mm f4 II and both 1.4x and 2x TC. The 300mm takes the TC the worst of the bunch. The 400mm I have is a pretty good copy I think, this is 7DII, 400mm, 1.4x III:
http://www.mcvoy.com/lm/wren.jpg
There is a HUGE difference in size/weight between the 600 and the 400. I use the 400 all the time, hand held, it's awesome (and as soon as I can I'll upgrade to the 400mm II). If you want to walk around and shoot then the 400mm II looks to be a very nice lens.
I know that the 400mm I varies in quality, I recently ordered what I thought was the mark II version from Adorama and they sent me the mark I version. I put it on and took a few shots and was freaking out because the quality was worse than the one I already own. (Also, had to push like crazy for them to take it back, B&H gets my money going forward.)
The little 300 is a sweet lens in how small/light it is. Also has a close focus that is unusual. The 400mm is bigger but still very hand holdable.
I've never held the 500mm but I suspect it is big enough you want it propped on something or mounted on a tripod. I find the whole tripod thing to be very limiting, I don't like it, so for me, the 400mm DO is just awesome. I wish they made a 600MM DO.
Good luck with your choice, my vote is the 400mm. Except I want one so buy something else
