I was a regular to the KTP in a Nissan X trail 4x2 (I say "was" because I am about to do KTP in my new Ford Ranger double cab woohoo... 4 weeks to go
) The X Trail just had too much National park mileage on it and needed to retire.
I did several trips in the X Trail and it peformed well.
The ground clearance is more than adequate
The packing space is great (if there are only two of you you can drop the backseat completely flat to create lots of space). Or only one of the split back seats.
Also bear in mind there is a Nissan dealer in Upington... I dont believe there is one for Kia. So if things go wrong... I know which make I would prefer to drive. Also Kia spares are less readily available. Often have to come from overseas. Having said that as you appear to be hiring, this may not be a worry for you but rather the rental company if it does go wrong
Nissan's are built tough... Kia is a bit like a 'wannabee' gravel road car. Its probably better off as a city vehicle carting kids to rugby practice..
I dont believe the Kia offers anything over the Nissan.
My X Trail did typically 8 litres to 100km (even when it had mileage on it)... so not heavy at all on fuel for a 2.0l vehicle carrying a load on gravel roads and even at higher speeds.
For me this is a no brainer.. Nissan X Trail
Watch out for the fridge ... it will kill your battery if you are not carefull. Personally I have always used a Coleman Extreme cooler or a CADAC 5 day cooler. You are going to want to take the items out of the vehicle fidge and put them in the fridge in your accommodation anyhow (unless you like push starting your vehicle every morning) so why not just use one of the above coolers with ice packs.. ours items stay frozen for long periods like this.
Try "Outdoor Warehouse" for the equipment you need.