I have long thought to trade my 300 for the 100-400 but just cannot part with this lens. It comes into its element at f/4 and as I have never used a tripod, the IS is handy.
Lovely photos! It seems we have access to the same subject material. I've also got lots of shots of Cockies, Lorikeets and King parrots, mostly taken with my 100-400.
I can see your problem. The 300/4 gives you an extra stop which is useful for hand-held shooting and blurry backgrounds. The 100-400 gives you f5 at around 250mm, f4.5 at 100mm, but f5.6 at 300 to 400mm.
However, on the issue of quality, I don't see that the 300/4 is necessarily better than the 100-400 at 300mm, according to Photodo results below. This is exactly the sort of situation where you would need to compare your 300/4 to the 100-400 at the same focal length. There would be no doubt, however, that the 100-400 would be better than your 300/4 if the subject required the use of a 400mm lens.
I don't find close-ups of wild life useful indicators of the quality of telephoto lenses because it's not always clear the degree to which the image might have been cropped and it's virtually impossible to duplicate the same shot with another lens for comparison.
The following images, taken with my D60 have all been cropped, so one is looking at an effective focal length well in excess of 560mm.
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