Consider the purpose of boot flex when choosing a new boot.
--When the lower leg tilts forward inside the boot cuff, the cuff may flex a lot, a little, or not much at all.
--When the cuff does not flex, all that pressure against the front of the cuff is delivered to the front of the ski (shovel).
--When the cuff does flex, that flexion lessens the amount of downward leverage the boot can deliver to the shovel.
--The softer the flex in the cuff, the less downward pressure on the ski's shovel. The stiffer the flex, the more potential downward pressure exerted. Leverage is what's going on.
--Delivering pressure to the shovel of the ski through leverage is what "getting forward" is all about.
--The skier can control the amount of leverage the boot delivers to the shovel but only if the boot flex is somewhat stiff. The skier chooses how far forward to hover the upper body - over the front of the boot or beyond. A stiffer flex gives the skier more options for how much downward pressure the shovel gets.
--A softer boot flex means that when the skier moves forward, some of that resulting shin-tongue pressure is absorbed by the cuff. Pressure will not be delivered as effectively to the ski.
--If you want strong control the forward pressure onto your ski's shovel, and to have the option of increasing that pressure or lessening it, choose a stiffer flex.
--Being able to control how much forward pressure you deliver to the shovel gives you more control over turn shape, and controlling downhill travel speed with turn shape is a part of strong skiing. If you use turn shape to control your downhill travel, choose a stiffer flex.
--The boot's cuff supports the skier's weight. A soft flex will not offer as much support. So if the skier skis fast, or skis in heavy snow, or skis in any way that delivers consistent and predictable high pressure against the shovel, a stiffer flex is called for.
--A skier who skis slowly on low pitch terrain with low edge angles, where this pressure will stay low, will not need the stiffer flex.
--Softer flex boots are cheaper. Stiffer boots tend to be more expensive for multiple reasons.
--A skier who is slow to react to unpredictable snow pressure changes will appreciate the softer flex. That soft cuff will absorb mis-timed body movements so as not to deliver erratic pressures to the shovel.
--A softer flex cannot be stiffened, neither temporarily nor permanently.
--A stiffer flex can be softened temporarily when needed. Install an elastic Booster Strap to replace the Velcro power strap, wear the Booster around the top of the liner inside of the plastic cuff, then loosen the cuff buckles a bit. The Booster will protect against shin-bang as the cuff is softened this way. Tighten the buckles to get the stiffness back.
--The purpose of soft boot flex is not comfort. If a stiffer flex hurts, then the boot does not fit. The boot's shell may be too wide, too high in volume, or too long for the foot. When any of these three parts of fit are too big, the foot and the shin can move around inside the boot and bang against the shell. It doesn't take much movement to cause pain. Changing the flex is not the solution. Changing the fit is.
--Paying for a stiffer boot includes free punching and grinding by the bootfitter. This is something the fitter does to the shell in order to create pockets in the shell that make room for bony protrusions on the foot.