Soft is not better for park, just as stiff is not better for all mountain.
The right flex is based on YOUR subjective needs based on body weight, ankle range of motion, strength, and what you have used in the past.
A boot that is too soft for you will be super inefficient, lack power, lack control, and increase your likelihood of shinbang. A boot that is too stiff for you will cause you be in the back seat more often and place your body in the wrong position.
Visit a boot-fitter and find out what the right flex is based on your body's needs and you will have a boot that skis awesome in the park, pow, and all mountain.
PS- all of our freestyle athletes are on 120 flex (or stiffer) boots.