Trip to Africa - Lens Suggestions - Canon 7D

I have a Canon 7D and will be going on a trip to Africa soon. We will be doing jeep tours so the subjects will mainly be landscape an wildlife.

I currently own a Tamron 75-300mm LD but am not all that impressed with it and it has no IS. I like it in manual mode, but its a pain after a short while.

I am currently debating between the Canon 70-300 IS USM and the Sigma 70-300 DG as possible alternatives. I have read mixed reviews on both so am looking for some insight.

Thanks!