Flex options provide comfort and performance depending on your preference and riding style. Generally, a stiffer boot is often preferred by advanced skiers while a soft flex is better for the average skier.
For further consideration:
Those with limited ability to flex their ankle (dorsal/plantar flexion), may prefer a stiffer boot. Also if you ski on groomed terrain using a carving ski.
If you prefer skiing in soft snow and ride skis with more tip and tail rocker you may prefer a softer flexing boot. Also for those that love to ski moguls.
The Dahu ski boot is a 3 piece/Cabrio design that provides progressive flexing. There is no starting or stopping point of the flex through the tongue.