If you want to see the Great Migration in the Masai Mara you need to go between the middle of July and the beginning of September. For the rest the Masai Mara offers throughout the year great wildlife sightings, only the vegetation will look different when going just after the rainy periods or just before. It will either be nice green or pretty dry, but both are very interesting for photography.
If you want to go to the Okavango Delta and to do water activities, you need to go during the time that there is enough water to do so. But also here the Okavango Delta is beautiful throughout the year.
If you think about going to South Africa you should know that the sightings in winter are often easier, because the grass is low and the animals gather around waterholes. In summer the bush is nice green and it is warm. It all depends on what is important for you and when you have the time to go.
In other words, always talk to the people you are booking your trip with to find out what the best would be for your interests and you will have a great photographic safari.
However, the bush is always amazing. You can let the bush surprise you with the beauty it offers at the time of your visit and you will go home with incredible photpographs.