FOsteology, I will try the best I can but there are a lot of variables involved. Are you shooting full frame or APS-C. With APS-C you get the benefit of narrowing your angle of view and have roughly a 540mm with your 100-400mm. With another lens you will have an 800mm roughly. Another variable is where your trip is at in Africa and is it with a photography tour or general tour? On a photography tour the vehicles you will travel in are designed to shoot through a stand up roof. Plus on photo trips they will turn off the engine to reduce vibration also. A tripod or monopod probably is not a possibility but a bean bag may be.
This is the trip of a lifetime possibly so taking the right equipment is obviously important. I obviously have seen and used the Tamron. I have seen images from the 150-500mm, they are good. I have not seen the 50-500mm however. On a photo specific trip non VC (VR) would probably be fine, but I would not recommend missing a shot because of movement. I can't honestly imagine the Sigma lenses being optically better but if the security of knowing you can get the shot is there, I would recommend the 150-500mm. A cost saving alternative could also be to get a 1.4x tele-converter for the 100-400mm.
I hope this has helped a little bit. Camera Land offers several versions of the 1.4x tele converter starting as low as $229. You will lose only a stop of light with the converter which is only a half stop at the maximum range vs. the Sigma. If you have any other questions please let me know. I look forward to some great images from Africa!
