I have no experience with Cape buffalo. But my guess is that any name brand solid would be fine and more or less act the same as any other brand. Straight and true through the targets. At least well enough that it wouldn't matter enough to make a huge difference. Regarding expanding bullets I really honestly believe, again, that most name brand bullets would suffice provided they were intended for "tough bullet" use. I seriously doubt that anyone can prove one way or another that one brand is really any better than another. I think your best bet is to see which will shoot the best of of your gun. I seriously doubt that an expensive bullet will perform any better than a cheap one (within reason of course.) Meaning I doubt that a 5 dollar bullet from Woodleigh will do better than a 1 dollar DGX from Hornady (I made the prices up, they're not real prices, just an example to make a point.)
And like Eaglesnester said, check with your Guide. He may at the very least steer you away from bullets that don't perform well if he doesn't have an idea to the best bullet available.
That said, pick the bullets you are the most comfortable with.