If it's for wildlife, the 100-400. If it's for something else, the 70-200 is a much sharper lens.
I'm using a 300 f/4L and i also have both TC's for it. It will still auto focus (allthough kind of slow) with the 1.4TC and give me 420mm. If a TC changes the lens to f/8, you will loose the auto focus unless you have a 1D series camera.(for example if you used a 1.4TC on the 100-400, you would loose auto focus.)
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