I'd definitely recommend the deluxe safari with the species in mind that you'd really like to see; you get to spend a lot more time with the bison, Przewalski horse, camels, and give you a lot better chance of seeing the waterbuck well. They can all be seen on the usual bus tour, but that is a lot more rushed. The vervets are seen on foot, so are usually not too tricky to view-- but then it does still come down to luck.