I've been on safari in Tanzania (bordering Kenya) and Southern Africa (visited South Africa but did our safaris in Botswana). Both were great experiences.
Cape Town is a wonderful city well worth visiting for 4 days or so. Robbin Island tells a great story, but there are many great things to see and do in the city itself, like its' great waterfront and Table Mountain. But there are no good game reserves here. The better game parks in South Africa are north and east, like Kruger.
Consider going on a trip, like we did, that covers visits to Cape Town, Safari in Botwsana (the Okavanga Delta and Chobe National Park), and a visit to Victoria Falls (straddling Zambia and Zimbabwe).
But I also know you would enjoy a trip to Eastern Africa (especially during the million animal migration). So maybe try one now, and in 5 or so years do the other.