Digital Lens

[MarkLG]

Hook

If you're going on safari and want to get some really good wildlife shots then you'll need something even longer than a 300, unless you can be sure you'll be able to get really close to the animals. You may become frustrated at missing shots due to the limited range.
The Sigma 50-500 is an excellent lens - although very heavy, and quite expensive. I owned one a while ago and used it extensively for motorsport photography. It's capable of producing some stunning shots in the right hands. In bright outdoor conditions you'll be able be to keep the shutter speeds up high enough to elminate lens shake - this is only a problem in low light. A monopod may be a better idea in these conditions - tripods take a while to get up. I never bothered with either.
Sigma also make a cheaper 170-500 lens which I haven't used so I can't comment on. Canon lenses in this sort of range are seriously expensive!
You're best option is probably to find a good second hand Sigma on ebay for the trip and then resell when you've finished with it - good lenses are very easy to sell on.