I spent 2 weeks between the Matetsi Gorge and the Hwange Safari Area in September and saw lots of buff, lion, eland, kudu, waterbuck, impala and klippys. All the good duiker and Gray's Rhysbok seem to have been shot out of my area by then, however, some nice ones were in the salt shed. Probably would be saying the same thing about the klipspringer if the previous hunters could shoot, the 5"er I got was shot at three times before over the summer I was told, but there were still several in the 4" class on the rocks, couple in the shed.
The late, dry season is about as mosquito-free (a.k.a. malaria) as it gets. I don't think I saw two or three of the blood suckers so was able to stop the meds when I got there. 100 degree temps, and the game is all about water... animals finding it and hunters carrying plenty of it.
That been said, for leopard or lion I believe I'd like April, as close to the first couple hunts of the year as I could get. Potentially bigger selection, more bait-friendly cats that haven't been hunter-educated in a while. That's what my PH tells me anyway when we were discussing it.