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One factor in favor of the 600II + 1.4XIII not mentioned so far is the MFD. This shot was with that combo and the 1DX cropped to image size 2865x2166 taken just outside the MFD but too close for the 800mm. For small perching birds and hummers, if you can get close the MFD plays a 'big' role in the image size and final quality. Maybe the 800 II would address this to some degree by having a closer MFD than version I, but not likely to get close to the MFD of the 600II. I guess if you can't get this close then the longer lens will be the best choice. Look forward to the 800mm II f4 - just need the antigrav unit to carry it around.