Guys, thanks for all the suggestions.
I'm travelling the first 2 weeks of March to Tanzania. This will be my fifth time to Tanzania. So far I have been shooting with APS size sensors.
Last year for example I was shooting with a 20D and a 30D with the following lenses: 17-85, 70-200 and a 300 f2.8 with a 1.4 TC. In practice I shot most good shots with the 300 and 1.4TC. This is really a very sharp lens!! (I attached a favourite shot of mine taken with this combination. It is so sharp that I have succesfully blown it up to to 40 by 60 inch...) Still I found myself sometimes in a situation that I wanted more reach. That's why this year I'm taking the 500mm.
But then I'm shooting this year with a 5D and a 40D. The 500 is a rental. I don't like the 100-400. I owned that lens, but it was never really sharp and I sold it. I do own a 300 f4 (old version, non IS) and maybe I'll bring this one to fill the gap. (means I'll be changing lenses though)
I did consider the 120-300 mm from Sigma. I'm travelling with a friend who is very happy with that lens. Maybe I can consider it, but I have the same dilemma: more investments...
Btw, I'm mostly shooting from a safari vehicle with a bean bag as support. With big lenses this is a very stable set up and actually I think you don't need a lot of IS.
Best regards, Giedo