The first day in Kruger a safari vehicle is used for the game drives. There is an optional excursion ($115) in the second day to use the safari vehicle or you could go on the bus which was included for the trip.
The disadvantage of the bus was it could only drive on hard surface roads. You had to shoot your photos through the bus windows which can result in glare and clarity issues. There is no radio contact between the bus driver and the other vehicles in locating lions, leopards, etc.
We used the safari vehicle for both the first and second days. We saw the big five on our first day. The lions and leopard were sighted when our vehicle was on gravel roads. Some of the sightings were due to radio contact between the drivers. On the second day we experienced a lion hunt but not the kill. The male lion and six female lions were walking single file through the brush about 100 feet from us. Our guide would drive ahead anticipating where they were heading. The female lions fanned out and the male lion stayed behind. One of the lions was within 10 feet from our vehicle and then took off very fast. All wildlife (impalas, Giraffes, etc.) in the area were running as fast as they could across the gravel road in front of us. I know this was a unique experience but a good guide, radio contact, and access to gravel roads will highly increase the odds from a safari vehicle. Also sometimes the animals make sounds which you will hear much better from a safari vehicle.
On our trip most of our group paid for the safari vehicle game drive experience on day two in Kruger and only a few people used the bus that day.
We used a bus (had no choice) for the optional Addo Elephant National Park game drive and I enjoyed it. In that park we could go on the gravel roads, there were very few safari vehicles to radio with, but taking photos through the bus windows still caused some glare and clarity issues.
Post Trip optional Chobe National Park morning game drive is with safari vehicle. The afternoon boat safari is on a two level boat.
If given the choice, I would always choose the safari vehicle option.