Gentoo - IMHO 300/2.8vr is the best lens when You need range and portability at the same time, and I'm not thinking about weight as app 1pound isn't that much once, as both lenses are very well designed. I'm writing about the length, which in many cases decides either You'll be able to take the lens on board or need to check it in. 200-400 is a long bastard :)
as for possible distances You can get:
300/2.8vr 200-400/4
n/a 200/4 - 200-400 is a winner
300/2.8 300/4 - 300 is a winner, probably the sharpest lens in whole Nikon collection
420/4 (TC14) 400/4 - I still would go with 300 as it gives You additional 20mm - IQ comparable
510/4.8 (TC17) 560/5.6 (TC14) - I would say tide - You gain more light with 300 and more reach with 200-400
600/5.6 (TC2) - 300 is a winner, 200-400 may not AF
so as You can see 300/2.8vr gives You more options, I would say that the only thing it lacks is zoom :) - as it may get handy, but OTOH once You are buying such a lens, You often have at least two slr's, so there's no problem with mounting the 300 + TC17 (I wouldn't go for the TC20 unless I would really need the reach) on one body and 70-200 (maybe with TC14) on the second one, as a backup camera in case You would need a shorter range.
As for using 300/2.8 as a 600/5.6, well it still has a better IQ than any other cheap (under $5k) lenses, maybe apart from Sigma 500/4.5 (but it's a little too dark once You add TC14).
