Well, the OP said his trip was going to be in May, so we should be hopefully hearing how things went. I suspect that both his '06 and .280 acquitted themselves adequately.
I tend to be in "Africa cartridges start at .30"" camp. It has nothing to do with the mythical difficulty of putting them down, but more with the psychology of taking $1500 shots. The rule on the hunts I have taken is "if it bleeds, you pay"--a lost animal costs you your trophy fee, just as if it had been recovered. I don't know about the rest of you, but that makes me anxious--I am not the kind of guy that would bet my buddies $100 or $500 that I would make a particular shot at the range, but trophy hunting in Africa is essentially a more expensive version of that. In addition, a lost animal can also mean that you don't get a second chance, if there is not another available on permit to your outfitter.
So I did what I could to minimize my risk of failure--practicing off of sticks, practicing quick second shots, using premium bullets and big cartridges, throttled down on speed. All these mean I am more likely to get a good first shot, maybe even a second hole, and hopefully an exit wound and its help in tracking.
I would love to take a 6.5x55 or a 7x57 on a cull hunt, but when a big trophy fee is in play, I like to bring my loaded dice.
