Difficult question ,Martin.I prefer the 400F5.6 to the 100-400.I just think it takes much sharper shots.Albeit when I look back at some of my early shots with the 100-400 they were okay.
The Sigma is a good lens,I quite possibly did not give it a fair trial.If someone is looking for a versatile lens with this focal range,and they do not already use the Canon lenses,or have not used them,then I think this lens is a good buy.
I did take a couple of reasonable shots with the Sigma.But,as my photography skills,re settings and technique etc,are somewhat wanting at times,I am not really the best person to get the best from the Sigma.
I do know that it is a vast improvement on the earlier Sigma,was it the 175-400.
I just wonder how "Condyk" ,the BF member who purchased the lens from me,has found the quality etc.
Also,I now only use the 100-400 when away from home,and for local sports etc,it folds down into a smaller size than the 150-500.
I only purchased the Sigma,as I do not do any more digiscoping,and I just thought the extra reach would be good.