Hi Steve,
I have done both trips and enjoyed them both but if I had to choose between the two then South Africa would be my choice every time.
Regards the US trip, I loved San Francisco and visited Alcatraz which was very interesting, Universal Studios was great fun. When I did the tour, you could fly down into the Grand Canyon by helicopter and then ride in a boat along the river but you cannot fly down into the canyon now. It's a choice of flying over it in either a helicopter or a plane.
Las Vegas was fantastic. I didn't go to any shows but just spent the evening(s) walking around looking at the free entertainment like the fountain show at the Bellagio and wondering around the different casinos soaking up the atmosphere. You have to go to Fremont streeet and see the light show! All in all it's a great trip.
The South Africa trip as a whole was brilliant! I had a great time and everyone in my group thoroughly enjoyed it. I must admit the main reason I went was to see the animals. I did all the game drives I could! We saw the big five and lots of other game. I recollect one evening sitting down to dinner and watching hippos having their dinner on the riverbank below us. Amazing!
If you can afford it then the Victoria Falls extension is fantastic. The hotel is in a game reserve, I woke up the first morning and there were zebras on the grass outside my room, and you are only 10 minutes walk away from the falls. As a hotel guest you have free access to the falls. Flying over the falls in a microlight is an experience I will never forget.
Regards
Jim