If I were you, I would go on the night drive. Not from a photographic perspective, but because there are some nocturnal animals that you stand a much better chance of seeing.
Photography on a night drive is certainly a challenge! But it can be done with a bit of trial and error
. Your flash could help to some degree, but personally I don't use mine unless there's an animal very close to the vehicle. On the game drives at Addo, the guide has a powerful spotlight and it is often sufficient light to get a decent shot like this one of a duiker...
Here's my method of taking such a photo
Firstly take a bean bag along to rest your camera on - this will allow you to use a slower shutter speed if the animal is staying still. On your camera, set the ISO expansion on (custom function 1.3) which will allow you to bump the ISO up to 3200 if necessary. As you'll probably be working with high ISO levels, turn on the high ISO noise reduction (custom function 2.2).
I found from experience that the camera metering wasn't accurate for this type of lighting (even spot metering), and so I simply set my camera into full manual mode, opened the aperture as much as possible (f2.8 in this case) and worked on different shutter speeds until I was happy with the exposure level. Obviously there's not always time to get things exactly right, but you'd be surprised by how quickly you learn what shutter speed might be required based on how bright the animal looks!
Your 50-150mm lens will probably be your best bet for this type of photography, but there's no harm bringing along the 50-500mm just in case. Moon and/or cloud cover really won't make much of a difference to you photos at night time - the moon simply doesn't provide enough light to be helpful even though we can see things better when it's shining brightly.