I have ruled out taking the 100-400 as it is too soft.
May be I'm quite biased about the 100-400, since it is a lens I adore. I'm afraid you get a bad example of it and that is a shame, since, I know that for personal experience in photosafari in SA, the 100-400 has the perfect zoom range for the business. By the way, what do you mean by 'soft'? Is the following image (100-400 at 300) soft for you? (Unfortunately I had to resize it to allow uploading it, loosing a lot of IQ)
P.S. I think the following article could be very useful.
I'm very well aware that the 100-400's are not all created equal. There are good ones out there and there are bad ones. I agree the focal length is awesome for a safari. But what made me stop using MY 100-400 altogether is when I tried to micro adjust it, the output was too soft that I wasn't even able to find the sharpest point. I do own the 70-300L also but I opted for the 70-200 f/2.8L II along with the 300mm f/2.8L II instead because I can put converters on both of these lenses, their faster and sharper.
As for TexPhoto's idea of zooming with feet, I'm just crazy enough to do that but I want to be able to shoot again the next day not being the days kill. I love a wide angle, I like getting right into my subjects face but they won't let me in Africa for some reason.
Actually they do, in many of the south african private game parks you can go on guarded hikes with several armed guards and spotters who are in radio contact so they can take you directly to the animals, i have never done one but would love to, they are definately not cheap and there is still more risk than shooting from within a vehicle however it allows you to get angles you simly cannot achieve from a vehicle. i'm going to look into it for a trip next year.