Copied from 'bird portrait' thread -
My non scientific assessment after a couple of days of using the Sigma 60-600mm S (and much longer use of the excellent Canon 100-400mm II):
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 60-600mm 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" and in case I know the reach is not critical, I'm using the 100-400 for its lower weight, easier handling and excellent quality.
W.