Did u read this? straigt from there site, should tell which one is for you.
The Booster Strap’s Choice:
The choice between the different models of The Booster Strap has to be done considering the following parameters:
The Booster Medium or World Cup fits Heavy/Tall athletes, Booster Soft fits Light/Short athletes
Medium or World Cup fits an aggressive skier, Booster Soft fits a quiet skier
For Tech events it is suggested to ski with Medium or World Cup, for Speed events with the Soft model
- Stiffness of Boot’s plastics
Stiff Boot → Booster Medium or World Cup, Soft → Booster Soft
- Stiffness of the Ski
Stiff Ski → Booster Medium or World Cup, Soft Ski → Booster Soft
It should be also considered that it is better to begin with a softer model rather thana stiff one. A softer model has a slower elastic response but makes much easier to feel the advantages of using The Booster Strap. Beginning with a too stiff model may not lead the skier to feel the difference.
In addition it has to be considered that there is an overlap between the models and their elastic properties. As an exemple, the properties of an high tensioned Medium model can be equal to those of a low tensioned World Cup model.