In the rainforest (I've trekked through them on two continents, so far...), light is often dim - you'll want f/2.8. Also, you're going to the rainforest - and the100-400mm is not a weather-sealed lens (and the push-pull zoom mechanism may suck the mist right in), whereas the 70-200 f/2.8 IS lens has weather-sealing. The Canon teleconverters are also weather-sealed.