Well, we can see why I'm having trouble choosing -- the "choice" is versatility versus ultimate IQ... Obviously, a great argument can be made for owning both, but money aside, the issue I see with that approach is my past history. That has been, EVERY time I had a similar choice to make with larger glass, the lens I left behind OR the one that was NOT mounted at the critical instant, was ALWAYS the one that would have been the better choice to use.
Current thinking is go for ultimate IQ with the realization that having 36MP does allow for a moderate amount of post cropping if necessary
Jack of course one can make the case for either of these two lenses endlessly. At the end of the day although one lens or the other may be missed in certain circumstances, if you analize the majority of types of shooting you'll do with the lens, it's moderately straight forward which one to select.
1. If you think you'll find yourself wanting or needing 600mm at any point, it's a no brainer (the 300 f2.8 with the 2x)
2. If you think you might fairly often shoot in low light, again it's the 300 f2.8
3. For relative compactness in size, esepcially while its in some sort of case or backpack, the 300 f2.8 gets the nod.
4. If you'll be shooting larger wildlife, then the versitilty of the zoom cannot be underestimated, therefore the 200-400.
5. For those times where you might be in close relative to your subject distance, I did fine the ability to zoom back to200mm a help.
6. If you know you would be satisfied staying in the range of 200-400mm in decent light and not often shooting subjects at great distances, I would definitely opt for the 200-400, regardless of slight light dropoff when shooting wide open. The lens comes into it's own within these parameters. Anything outside this rannge, and performance can precipitously drop off and herefore would make the 300 f2.8 plus teleconverters the prefered choice.
Dave (D&A)