Hi BC
I have the Sigma 120-400 and to be honest I tried the 100-400 on my camera and did not notice a difference in speed and focus, 2ndly what I have seen the 2nd hand price of the 100-400 is nearly the same as a new one.
What I suggest is take you're camera with when deciding as you know the speed and focus of what you have and decide from there, try both
The 150-500 has that extra reach which sometimes comes in handy but the neg is if the animal comes too close
![Dunno -O-](./images/smilies/icon_nixweiss.gif)
diffecult
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