Hi Jan,
Sorry I've just seen your question now - Zambia was the icing on the cake for me mainly because I had a brilliant time in South Africa and yet the add-on was even better! As our tour manager described it, a real "out of Africa" experience. I loved the Zambezi Sun hotel, with wildlife in the grounds (I think you would stay in the same hotel - it's had a rebranding since). Everything about the add-on was just wonderful - being able to walk to the Falls in our (very minimal free time) and explore for ourselves; the sunset boat trip on the Zambezi with a fiery red sky and sundowners; a gourmet meal on the Royal Livingstone Express steam train, the helicopter ride over the Falls (I wasn't brave enough for the microlight!). It was all so enjoyable - and we didn't even have the option of the cheetah experience (although we did something similar in South Africa).
Regarding when to go, I was there in late Sept/early Oct and while the water level was lower than the wet season, the Falls were still very impressive, especially from the air. The advantage was that due to the lower levels, we could walk up to the very edge of the Falls (you have to keep your distance during the wet season, as the spray I gather is immense). So it's up to you what you'd prefer.
All the best,
Bob