What to do? What to do?

We took a niece and her family to South Africa last August - 7 nights in Kruger public camps, self drive, and 3 nights in a private camp.  I took both the 40-150mm f/2.8 (with a 1.4x TC) and the 100-400mm and shared them with my nephew.

During the self drive part of the safari the 100-400mm was definitely the most useful lens and was essential for birds.  It was only in low light in the very early morning and late afternoon that the 40-150mm was essential.

In the private camps everything was much closer, a lot of the shooting was done in low light and we didn't pay much attention to birds.  We both preferred the 40-150mm.

On the seven safaris that I have done I have very rarely come across a situation where 40mm was too long.   One time in a private camp a female lioness walked right up to the side of our open top vehicle but I had no time to lift the camera and take a shot - and no intention of attracting her attention anyway.  On another occasion, at night, we can across two male lions on a buffalo kill and, in a surreal scene lit by red light from the driver's spotlight, a 50mm f/1.8 on an APS-C DSLR wasn't wide enough, but that was the only fast lens that I had with me.  Having said that, 12-40mm is a useful lens for early morning sunrise shots.

I don't know where you are going on safari and for how long.  If you are going to East Africa or Kruger NP public camps, the 100-400mm will be the best lens.  In South African private camps there will be situations where only the 40-150mm f/2.8 will be usable.  I can't speak for Botswana.  I would try to take both lenses if you can.