Well I have to agree with the above posts that suggest you do both if possible, each will give you a completely different but equally amazing experience. Not sure I agree about the zoo bit though, our experiences [a mere 4] in Sabi Sand have been incredible and certainly not zoo like, if that was the case you would not spend all that time driving around looking for wildlife. There is radio contact between the vehicles but that usually enhances things, ensuring you get to see the best sightings.
We absolutely love KNP but as long as we can afford to we add a few days in a concession at the end to enhance our trip even more. Another consideration is distance, the time it takes to travel from home to SA makes you want the best 'bang for your buck' so to speak.
We have had some great sightings in Kruger, lots of plains game, heaps of elles, rhino, zebra, giraffes and many lovely birds and so much more. We head out the gate at opening time each morning and stay out till the gates closed. We have had a few lion sightings, one leopard sighting, lounging in a tree, but unfortunately we did not get much of a look due to the three lane traffic jam on the adjacent road ! But our next trip we are staying much longer and hope to see a lot more, as you can see reading these forums the more time you have the more you see! A great sighting was a pack of seven wild dogs, trotting down the main tar road near our camp, and another early morning on leaving camp there was a magnificent male lion reclining in the middle of the road, he seemed completely unperturbed as all the cars made their way carefully around him.
Now as for our stays in the adjacent Sabi Sand reserve, what made these stays truly outstanding was the big cat sightings. Last trip we lost track of the number of leopards and lions we saw, on our first drive we came across a mother and daughter leopard together, then a leopard with a kill up a tree, then watching it go to a puddle to drink, saw leopard and lion virtually every drive. Also saw a beautiful serval and magnificent cheetah, a 4 week old leopard cub with Mum that we had all to ourselves due to strict protocol. But the highlight and most amazing experience we have ever had was to witness a kill late one night, lion taking an impala. As an animal lover I never thought I would want to see this but it was truly awesome and took our breath away.
So for those planning their trip who ask for opinions on staying at a combination of KNP and concessions we would have to say if you can do both then go for it, they are completely different. While of course it all comes to luck on the day, all the best sightings we had were off road and found with the expertise of our and other rangers, there was never more than 2 vehicles at a sighting, and there was definitely nothing ‘staged’. We had to search for the animals, they certainly did not just appear. Great care was always taken not to disturb any animal in any way.
So if we could go back tomorrow and could afford it, we would probably do much the same thing again. Driving through Kruger Park is fantastic and finding game yourself in your own car is certainly a thrill, we did plenty of research beforehand and it paid off with many sightings. There is nothing quite like having to stop the car for an elephant or giraffe family to cross the road. Then the very different private lodge experience of being driven by a top ranger [and tracker] with the most amazing amount of knowledge about every aspect of the bush, and being able to answer any question, well that combined with some truly amazing big cat sightings, just blew us away. That is of course without any mention of the people, food, accommodation etc that you get [and pay for!] in a private lodge.
Good luck planning.
Croc Bridge 27-29 October
Skukuza 30 oct -5 November
Satara 6-10 November
Olifants 11-12 November
Greater Kruger 13 -18 November