The answer to this question very much depends on the time of year. Gansbaai has very good Great White sightings year round.
Simons Town is more seasonal due to the behavioural habits of the seals on Seal Island. The best time to see Great Whites and their famous hunting habits around Seal Island is June – September.
Chris and Monique Fallows of Apex Predators (http://www.apexpredators.com) offer superb trips to False Bay. You can check the monthly updates on their website where they detail what has been going on in the waters with their "Sharkbites" newsletter.
Outside of these months your best option is Gansbaai and, I recommend Brian McFarlane who operates Shark Cage Diving.Net (http://www.sharkcagediving.net).
Edited: 13 years ago