I just went through a similar decision: I'm travelling to Costa Rica and Peru this spring and was trying to decide what lenses to bring. For the rain forest I thought the 70-200 would be the ideal focal length (and I'll probably bring the 1.4 extender to add a little reach if necessary). I went with the f4 though, but mainly for $$ reasons (but the extra size/weight can be an issue if you plan to do much hiking...it gets very hot & humid in the rainforest [:P]). As for the 100-400,the range is great for any wildlife, butI wouldn't want to use a push/pull zoom in any humid/misty place.


Myrecomendation is to go with the 70-200 (and maybe pair it with a tele for more reach and not much extra weight).


Stephen