Well I guess, The SuperAmerica is much more modern and updated than that of the 575.
According to "Motor Trend Magazine" they say:
The new car is called the Superamerica. Is that a great name, or what? It's as brazen and comic-book perfect as "Captain Fantastic" or "Speed Racer." And, yes, this is a car that could get away with a giant "S" on its hood: It's an even more potent, alfresco version of the awesome 575M Maranello. In fact, pointing to a top speed of nearly 200 mph, Ferrari claims the Superamerica is now the fastest production convertible in the world.
In myopinion,
When the SuperAmerica got released to the public for purchase, they didn't make much fuss, as much fuss that is, for instance to the "Enzo" or "F430".
The engine is also at the front, and not at the back, and although it looks brilliant, It doesn't make that VROOM that other Ferraris have such as the F430 or the Enzo.
Anyway, your decision, either way 575 or SuperAmerica both great Ferrari's in different ways.