International Internet based start-up
Where should we be located?
What business type will be best for us? A Joint Venture with an existing in the US; Register in South Africa and Plant a branch in the US; New Company in the US; Something else?
To sum it up, we want to take the option that is going to meet the sufficient standards of our business, be cost-effective and will take the least possible amount of time to set up.
Your description of this future business is quite broad, so I will try to give you a general answer that would cover all corners.
Although depending on what your activity in South Africa would be (development, customer service, etc), since all the shareholders are located in South Africa you will most probably be required to register your company there (although the details of company registration in South Africa are beyond my knowledge).
Then, since your customer base is in the US you might benefit from also registering a company in the US as well. This also greatly depends on the extent and nature of you business.