Travelgeek, in her reply to the OP regarding safety, in the thread on the OAT web site, asserted the following:
"If you are at all interested in the variety of peoples and cultures of South Africa, then I recommend the GCT Highlights of South Africa trip. I have not been on this trip, but have visited some of the same areas. Unless, you take the pre-trip, there are no overnight stays in game parks, but you do get to visit Kruger National Park. This trip will expose you to the wild life of Africa, the people of South Africa as well as the varied topography of a very beautiful country without staying overnight in a game park unless you do the pre-trip. You can go to Victoria Falls on the post-trip. This trip will give you a glimplse of Africa and help you decide which trip is right for you once you do return to Africa. As anyone who has ever been to Africa will tell you, once you've been to Africa, you will return. "
We concur that Travelgeek's comparison is a good one. Other differences are in group size( OAT groups are significantly smaller ), itineraries( OAT INCLUDES Victoria Falls, and possibly, cost, when one includes optionals and pre-and post-trips to make the trips comparable. Simply put, if your emphasis is cultural, then the Grand Circle-branded trip is for you; if your emphasis is on an adventurous safari-type experience, where you'll see loads of animals, then take the OAT-branded trip.