Wiebe - first: congrats for your new lens!!!
Concerning the comparison of these two lenses - if the distance to the bird is +/- same, you have more pixels engaged in the detail with the 600mm. And you will have even more if your next lens is 800mm (

- dream, at least for me. I just don't know how I could handle such a monster).
Hi Ilko, thanks... as to comparison - the location / angle / distance were all identical (to the nearest 0.1m

) so indeed 50% more linear detail to be captured. Shooting conditions / settings were different though (high ISO on the 100-400 shot, causing some loss of contrast / detail).
My non scientific assessment after a couple of days:
100-400mm II can achieve the best detail / sharpness - if you can get close enough for good framing
60-600mm comes quite close in sharpness and contrast while adding reach for tight framing - if you can handle the weight or work from a tripod / monopod (the additional kilo of weight, way in front of the camera, makes itself felt...)
100-400 II + 1.4xTC III (so 560mm f/8) lags behind a bit in contrast / sharpness (all on my 7DmkII)
So this new lens will be used on occasions reserved for photography, where I know I'll need tightest possible framing (or need the massive flexibility in framing of the 10x zoom). When hiking "with the possibility of capturing a nice shot" I'm using the 100-400 for its lower weight, easier handling and excellent quality.
W.
P.S. I'll try the 60-600 + 1.4xTC for the 840mm reach while on holiday - see what result I can get with that

(on crop body so >1200mm equivalent

)