there are major differences between Grand Circle and OAT trips, in particular when the travel to the same locations. In general, the OAT trips are more adverturous, takeing in sites which require better physical condition, more agility and on occasion, more walking, over terrain which may not necessarily be even.
The African trips are different in the exception that they offer more time to view wildlife rather than cultural sites in cities. You don't need physical agility for the OAT African trips, because they don't let you do any walking beyond going from your tent to the lodge and back again. They don't let you out of the vehicles, save for a bathroom stop using the "loo with the view." Hence there's literally NO walking.
The major difference, then is the type of experience you want. If your desire is limited to wildlife, then the OAT trip is by far the superior trip. OAT's Ultimate Africa, with the pre-trip extension to Kruger is without a doubt excellent. This trip includes Victoria Falls as part of the base trip, saving you as compared to the Grand Circle trip. The Grand Circle branded trip trip, however, is cheaper. If your desires are to see clutural sites in cities, with some safari game drives, then you may be satisfied with the Grand Circle branded trip as opposed to the OAT branded trip.
At any rate, the choice between trips depends on what you want from the trip.