Peter Capstick spent a lot of time in Africa and his book on guiding spends an entire chapter on this. He acknowledges both sides, but makes some really interesting points: (a) You better be able to handle the big cartridge you're shooting or it'll do you little good, and (b) if you really can pinpoint your shot, the smaller calibers will in fact do the job.

For me, I reckon I'd want something that I know for a fact I can put in the kill spot....and something too that if faced with a charge, will finish the job. Like Capstick said, "It's one thing to hit the kill zone from a resting position at a safe distance; make sure you can repeat that feat at a distance measured in feet if it occurs."