Definately go to cape town! Tickets are cheap with internal airlines such as kalulah, one time and mango airlines. Cape Town is much more scenically beautiful where johannesburg is more a business district. However, from Johannesburg you can go up to Mpumalanga and Kruger national park (4 hour shuttle ride) where there are safari areas. I wouldn’t spend much time in johannesburg unless you know people there. Hope you enjoy your trip!